Benvenuti, Cristoforo

Chiggiato, P

Cosso, R

Hellgren, N

Lacarrère, D

CERN Document Server

EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHORGANISATION EUROPEENNE POUR LA RECHERCHE NUCLEAIRECERN MT/95-05 (SM)Characterization of the Internal Surface of Superconducting 352 MHzAccelerating Cavities using a dedicated Scanning Auger SpectrometerR. Cosso, C. Benvenuti, P. Chiggiato, N. Hellgren, D. LacarrèreCERN, CH–1211 Geneva 23, SwitzerlandAbstractThe energy upgrade project of the Large Electron Positron (LEP) collider at CERN from55 to about 90 GeV requires the use of about 250 superconducting four–cell 352 MHzcavities. The majority (over 200) of these cavities, which are being manufactured bythree European industrial firms according to a procedure developed at CERN, are madeof copper, coated internally with a 1.5 µm thick niobium film deposited by magnetronsputtering.Contamination and defects, which may be produced during the manufacturing process,have detrimental effects on the cavities' RF performance. For this reason, a dedicatedcomputer controlled surface analysis instrument, incorporating Auger ElectronSpectroscopy, Scanning Auger Mapping and secondary electron imaging, has beendesigned and built at CERN, to perform elemental and microscopical characterization ofthe LEP-type cavities surface.In this paper a study of the inner surface of a few of these cavities is presented, includingthe characterization of the defects found. In addition, the variation of the surfacecomposition of two copper cavities, prior to niobium deposition, after bakeout cycles attemperatures in the range from 120°C to 200°C is reported.Paper presented at the "6th European Conference on Applications of Surface and InterfaceAnalysis (ECASIA '95')", Montreux, Switzerland 9-13 October 1995Geneva,  Switzerland29 November 1995Characterization of the internal surface of superconducting352 MHz accelerating cavities using a dedicatedscanning Auger spectrometer. R. Cosso, C. Benvenuti, P. Chiggiato, N. Hellgren, D. LacarrèreCERN, CH–1211 Geneva 23, Switzerland1. IntroductionThe energy upgrade project of the Large Electron Positron (LEP) collider at CERN from 55to about 90 GeV requires the use of about 250 superconducting (SC) radio frequency (RF)352 MHz four–cell cavities [1], which are being progressively installed in the existing LEPmachine. The majority (over 200) of these cavities, which are being manufactured by threeEuropean industrial firms according to a procedure developed at CERN [2], are made ofcopper, coated internally with a 1.5 µm thick niobium film deposited by magnetronsputtering. Coated cavities provide several advantages with respect to bulk niobium cavities,especially in terms of thermal stability and cost. Contamination and defects which may beproduced during the manufacturing process have been shown to have detrimental effects onthe RF performance of the cavities [3,4]. For this reason, several diagnostic techniques havebeen developed and implemented to characterize the inner surface of SC cavities [5],including scanning thermography [6] and scanning electron microscopy [7]. A dedicatedsurface analysis instrument, incorporating Auger Electron Spectroscopy (AES), ScanningAuger Mapping (SAM) and Secondary Electron (SED) imaging, has been recently designedand built at CERN [8, 9], to perform elemental and microscopical characterization of theLEP–type cavities surface, possibly providing a direct link between the nature of the localsurface defects and/or contamination and their effects on the RF operating conditions.2. Results and discussionThe cavities, produced in industry, are accepted or rejected by CERN following RF tests.About 160 cavities have been accepted to date (september 1995). The rejected cavities aresystematically inspected by means of a video camera set–up on a computer controlled opticalinspection bench [10]. The results of the optical inspection and of temperature mappingmeasurements provide, in most cases, an explanation for the inadequate performance of thecavity. In some cases, however, further investigations using the surface analysis dedicatedinstrument are necessary to provide information on the nature of the defects in the Nb film.Six Nb coated cavities have been analysed to date. In general, the non defectiveareas of the Nb film are oxydised (Nb2O5) [11] and present very low carbon contamination.The majority of the surface defects consist of regions where the Nb film is missing; thesubstrate Cu surface is then visible, usually oxydised and contaminated with C (examples inFig. 1a, 1b, 1c). This kind of defect was found 12 times, with linear dimensions of theexposed Cu area ranging from ~50 µm to ~1 mm. In one particular case, an area ofapproximately 15 mm x 4 mm of the film was missing. In three cases, besides Cu, Ni and Fewere also detected in the defective area (example in Fig. 1d, 1e, 1f). In general, it has beenpossible to correlate the number and the total surface area of these non superconductingdefects with the unsatisfactory RF performance of the corresponding cavity.a)100µm b)100µm c)200µmd)300µm BA..e)300µm 100 300 500 700 900Kinetic energy (eV)ABf)g)400µm h)150µm i)150µmFig. 1: Examples of typical film defects. The convention used for the Auger maps is thathigher concentrations of the detected element correspond to lighter points; a) SEDimage of a ‘blister’ type defect; b) Nb SAM corresponding to a); the film is broken andthe Cu substrate is visible (dark area); c) Cu SAM of a large defect where the Nb filmis missing; d) SED image of a defect ; the corresponding SAM of Nb (e) revealsinhomogeneities of the Nb concentration; the Auger scans taken at points A and B,shown in f, reveal the presence of Cu (A) and of Ni and Fe (B); g) ‘etching’ pits, SEDimage; h) SED image of a ‘stain’ of the Nb film; the corresponding Cu SAM is shownin i. The Nb film is not missing (as seen from AES measurements) but some Cu isdetected.Other types of defects were found, such as:– “Etching pits” (Fig. 1g), attributed to gas bubbles retained on the Cu surface during thechemical polishing [4]; in one of the inspected cavities hundreds of these pits were found inone of the cells; however, they seemed to be uniformely coated with the Nb film.– “Stains” on the Nb film (Fig. 1h, 1i), where besides Nb a small Cu signal is detected viaAES, indicating either partial film peeling off or diffusion of Cu from the substrate; this kindof defect was found 3 times.– Carbon granules, where a strong, local C contamination was found (4 times).– Dust particles, detected as charging particles by the instrument (~10 times).This classification is not exhaustive but is representative of the kind of defects of the Nbcoating encountered to date in the characterization of industrial cavities. It can be concludedthat the main problems are related to the film adhesion; the careful preparation of the Cusubstrate seems to be of crucial importance, since residuals of chemical products or foreignmetal particles can endanger the adhesion of the film and result in unsatisfactory RFperformance of the cavity.For this reason, a study of the residual surface contamination of an oxygen freecopper cavity prior to niobium deposition, before and after bakeout cycles at differenttemperatures (120_C < T < 200_C) was carried out. Outgassing measurements wereperformed during each bakeout cycle using a calibrated Residual Gas Analyser. The numberof outgassed molecules per square centimeter was calculated for the most important gasspecies desorbed. The same experiment was performed on a Cu cavity from which the Nbcoating had been chemically removed (‘stripped’), to compare the results with the nevercoated (‘new’) cavity and to get an indication of the role of the film in “gettering” impuritiesfrom the Cu substrate.Average numbers of outgassed molecules during each bakeout cycle are shown inFig. 2a and b for H2, H2O and CO, CO2, respectively. These gas species account for thelargest part of the total number of molecules desorbed.AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA0123451 2 3 4H20/newH2/newH2O/strippedH2/strippedBakeout cyclea)AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA0123451 2 3 4CO2/newCO/newCO2/strippedCO/strippedb) Bakeout cycleFig 2: Number of outgassed molecules during each bakeout cycle; first bakeout at 120_C,followed by three bakeouts at 200_C. a) H2 and H2O; b) CO and CO2.The largest outgassing, especially in the new cavity, takes place during the firstbakeout at 200_C. The decrease of the number of oxygen containing molecules released,between the first and second bakeout at 200_C, is more important in the new cavity. Thehydrogen outgassing is higher in the new cavity and shows little change between thebakeouts at 200_C for both cavities. The outgassing of CO and CO2 during the first bakeoutat 200_C is much higher in the case of the new cavity; during the following two bakeouts itis lower for the new cavity than for the stripped one. This agrees with the observation that theO concentration (measured by AES) after the first bakeout at 200_C is lower in the newcavity; the probability to form CO and CO2 molecules is therefore smaller.The main results of the AES measurements can be summarized as follows:i) Before the bakeouts, the carbon content was higher and the oxygen content lower for thestripped cavity; the overall Cu concentration was similar for the two cavities.ii) The new cavity presented an important chlorine contamination, which decreased aftereach bakeout cycle at 200_C, whereas in the case of the stripped cavity very little Cl wasfound. Such a high Cl contamination was not observed during AES measurements performedon Cu samples using a 'traditional' spectrometer. The origin of this contamination, and itspossible correlation with Cl outgassing measurements is currently under investigation.iii) The Cl and C contamination level increased after the first bakeout at 120_C anddecreased after subsequent cycles at 200_C in both cases. The oxygen level decreased aftereach bakeout cycle.iv) The impurity content on the surface of the new Cu cavity was not uniform. In particular,the Cl concentrations presented large variations; in some points very high C concentrationswere detected. This could possibly endanger the local adhesion of the Nb film. The surface ofthe stripped cavity was generally more uniform.v) Some suphur segregation was observed in both cases, increasing slightly after eachbakeout cycle at 200_C.3. ConclusionsFurther experiments are planned using the dedicated surface analysis instrument according tothe needs of the cavity manufacturing and maintenance programme. The consequentlygrowing statistics of typical surface defects will hopefully help establishing correlationsbetween these and the cavities RF performance.4. References[1] C. Wyss, Proc. of the “Particle Accelerator Conference”, San Francisco (1991) 150.[2] C. Benvenuti, S. Calatroni and G. Orlandi, Physica B 197 (1994) 72 and referencestherein.[3] D. Bloess, Proc. of the “3rd Workshop on RF Superconductivity”, Argonne, USA (1987)359.[4] S. Calatroni et al., Proc. of the “6th Workshop on RF Superconductivity”, NewportNews, Virginia, USA(1993) 687.[5] C. E. Reece, Proc. of the “3rd Workshop on RF Superconductivity”, Argonne, USA(1987) 545 and references therein.[6] Ph. Bernard et al., Nucl. Instr. and Meth. 190 (1981) 257.[7] A.G. Mathewson and A. Grillot, J. Vac. Sci. Technol. 21 (1982) 867.[8] D. Lacarrère et al, Proc. of the “6th Workshop on RF Superconductivity”, NewportNews, USA (1993) 702.[9] C. Benvenuti et al., submitted to Nucl. Instr. and Meth.[10] E. Chiaveri et al., Proc. of the “Particle Accelerator Conference”, Dallas (1995) andreferences therein.[11] J. Halbritter, Appl. Phys. A 43 (1987) 1 and refs. therein.

Characterization of the internal surface of superconducting 352 MHz accelerating cavities using a dedicated scanning Auger spectrometer

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