1000 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 35, NO. 4, AUGUST 2007 Purity of Nb and Pb Films Deposited by an Ultrahigh Vacuum Cathodic Arc Jerzy Langner, Marek J. Sadowski, Pawel Strzy˙ zewski, Jan Witkowski, Sergio Tazzari, Luciano Catani, Alessandro Cianchi, Jerzy Lorkiewicz, Roberto Russo, Jacek Sekutowicz, Tadeusz Paryjczak, and Jacek Rogowski Abstract—This paper reports on recent progress in the applica- tion of ultrahigh vacuum arc technology, which was proposed as an alternative solution for the deposition of thin superconducting films of pure niobium upon the inner surfaces of RF cavities designed for particle accelerators. New experiments were con- ducted to deposit superconducting films of pure niobium and lead needed for the modern accelerator technology. Presented scanning electron microscopy, scattered ion mass spectroscopy technique, and glow discharge–optical emission spectroscopy studies of such produced Nb and Pb films showed that the concentration level of impurities is lower than 0.2% and 1%, respectively. Achieved cleanliness goes together with outstanding superconducting prop- erties. The main experimental results and characteristics of arc-deposited thin superconducting films are discussed, and the progress achieved recently in the formation of such films is presented. Index Terms—Photocathodes, superconducting accelerator cavities, superconducting films, vacuum arcs. I. I NTRODUCTION V ACUUM ARC technology is widely used in scientific laboratories and industry in order to produce various surface coatings. The capability for implementation of various metals as a cathode material as well as for operating the arc in various reactive gas environments makes this technology suitable in producing desired metallic, nitride, oxide, or carbide coatings. Desired surface layer can be formed on constructional elements, even those having complicated shapes. High adhesion and corrosion resistance can be achieved. In comparison with sputtering technique, where atoms have an energy of a few elec- tronvolts, vacuum arc discharges can produce ions with higher kinetic energies, ranging roughly from 15 eV to about 150 eV [1]. It results in the formation of a denser film and strongly Manuscript received May 17, 2006; revised January 3, 2007. This work was supported by the European Community-Research Infrastructure Activity under the FP6 “Structuring the European Research Area” program CARE under Contract RII3-CT-2003-506395. J. Langner, deceased, was with Andrzej Soltan Institute for Nuclear Studies, 05-400 Otwock-Swierk, Poland. M. J. Sadowski and P. Strzy˙ zewski are with Andrzej Soltan Institute for Nuclear Studies, 05-400 Otwock-Swierk, Poland (e-mail: p.strzyzewski@ ipj.gov.pl). S. Tazzari, L. Catani, A. Cianchi, and J. Lorkiewicz are with the University Tor Vergata and INFN Roma 2, 00-133 Rome, Italy. R. Russo is with the University of Napoli and INFN-NA, I-80078 Napoli, Italy. J. Sekutowicz is with DESY-MHF, 22-603Hamburg, Germany. T. Paryjczak and J. Rogowski are with the Technical University of Lodz, 90-924 Lodz, Poland. 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/TPS.2007.896926 reduces voids and columnar growth. All these features arouse the interest of technologists. Nevertheless, the drawback is the creation of microdroplets. They are formed near the cathode region, transported to the target, and next become embedded in the film increasing its roughness. In order to eliminate the microdroplets from vacuum arc plasmas, one can apply various magnetic filters. The general principle of their operation is to separate the microdroplets from the plasma stream by means of magnetic field. Separation is achievable because microdroplets, due to large mass, move from the cathode almost linearly in contrary to single ions and small charged aggregates, which undergoes deflection in the magnetic field. In R&D programs concerning the construction of large lin- ear accelerators, particular attention was paid to the possibil- ity of depositing thin superconducting layers. Originally, the magnetron sputtering has been proposed as a well-established deposition technique [2]. Unfortunately, some unwanted fea- tures of the layers deposited in that way appeared to be impos- sible to overcome (e.g., Q 0 factor degradation, insufficient film quality and purity). Therefore, a new concept, deposition of thin superconducting layers by arc discharges under ultrahigh vac- uum (UHV) conditions, was proposed several years ago [3], [4]. Considerable progress in the development of this technology has been achieved recently. It consisted of an effective elimina- tion of the microdroplets in planar-arc facilities. Our previous report [5] describes two types of the magnetic filters suited for UHV conditions. The water-cooled filters work very stably for long duration. In addition, it is possible to bake out the inner surface of the filter in order to achieve the UHV conditions. Experimental and engineering studies are still continuing in order to improve the efficiency of microdroplet filtering and plasma transport through the magnetic channel. Another paper [6] has been devoted to residual gas analysis in the vacuum chamber under the UHV conditions: before, during, and just after the vacuum arc deposition. Moreover, a comparison of gas compositions between HV and UHV has been presented. Such an analysis is necessary when very clean layers of metals are needed. As an example, in our case of superconducting niobium films on a substrate heated to 150 ◦ C, a careful residual gas treatment allowed to reach the value of residual resistivity ratio (RRR) equal to 80 [7]. RRR is defined as the ratio of the resistivity at a room temperature to that measured at 10 K. Filtered cathodic UHV arc technology opens a new road to many applications where very pure metallic and/or superconducting films are needed [8]. This paper reports on recent efforts undertaken together by Tor Vergata University and Soltan Institute for Nuclear Studies 0093-3813/$25.00 © 2007 IEEE