A Review of Niobium (on Copper) Sputtering Technology S. Calatroni

9 October 2006 Sergio Calatroni - CERN - Sputtering Technology2 Why films: advantages / disadvantages Residual resistance: topics for further R&D : –Effect of roughness –Effect of film structure –Effect of hydrogen –Effect of surface oxidation Conclusions and perspectives Introduction

9 October 2006 Sergio Calatroni - CERN - Sputtering Technology3 Why films Advantages (primary objectives) –Thermal stability –Cost –Innovative materials Advantages (learned from experience) –Optimisation of R BCS at 4.2 K (sputtered niobium films) –Reduced sensitivity to earth magnetic field Disadvantages (known from the beginning) –Fabrication and surface preparation (at least) as difficult as for bulk Disadvantages (learned from experience) –Deposition of innovative materials is very difficult –Steep R res increase with RF field (sputtered niobium films)

9 October 2006 Sergio Calatroni - CERN - Sputtering Technology4 The surface resistance can be written in the form: R s (H RF, H ext, T) = R BCS (H RF, T) + R fl (H RF, H ext, T) + R res (H RF )  The dependence of R BCS (0,T) on has been verified by changing the sputter gas  R BCS (H RF, T) has an intrinsic dependence of H RF  R fl (H RF, H ext, T) has a dependence on similar to R BCS (0,T) The surface resistance

9 October 2006 Sergio Calatroni - CERN - Sputtering Technology5 R BCS at 4.2 K Nb bulk: ~900 n  Nb films: ~400 n  R BCS at 1.7 K Nb bulk: ~2.5 n  Nb films: ~1.5 n  Theoretical and experimental BCS resistance at zero RF field Nb bulk

9 October 2006 Sergio Calatroni - CERN - Sputtering Technology6 R fl = (R fl 0 + R fl 1 H RF ) H ext Fluxon-induced losses at 1.7 K are characterized as R fl = (R fl 0 + R fl 1 H RF ) H ext The minimum values are obtained using krypton as sputter gas: R fl 0 = 3n  /G R fl 1 = 0.4 n  /G/mT Triangles: bulk Nb Squares: coatings on oxide-free copper Circles: coatings on oxidized copper    R fl 0 [n  /G] (a) Fluxon-induced losses I

9 October 2006 Sergio Calatroni - CERN - Sputtering Technology7 Nb/Cu films – categories There are two categories of films –Films which are intrinsically films Thin, small grains, microstrained, under stress Problems: defects & microstructure, (impurities), surface state Examples: magnetron sputtered films on oxidised copper –The trend among workers is to aim for films which are bulk-like Thick, large grains Problems: hydrogen, surface quality Examples: high-energy deposition techniques, annealed films, (Nb Cu-clad) Of course a film from one family may as well present all the problems typical of the other family…

9 October 2006 Sergio Calatroni - CERN - Sputtering Technology8 Research lines around the world Effect of substrate: roughness, thermal impedance –Non uniform coating, H enhancement (demagnetization), increased granularity. Thermal feedback  Optimisation of substrate preparation (electropolishing), study of angle-of-incidence effects, conformal coatings. Measurements of K Film structure – defects –H c1 reduction, hysteretic losses  Towards a bulk-like film: bias sputter deposition, high-energy deposition techniques, high-temperature annealing of films Effect of hydrogen –Hydrides formation  Measurements of H 2 contents, outgassings Oxidation –Localized states, corrosion of grain boundaries  Al 2 O 3 cap layers

9 October 2006 Sergio Calatroni - CERN - Sputtering Technology9 Electropolishing – Polarization curve Production of Cu(OH) 2 on the surface! Production of O 2 bubbles! Electrical resistance seen by the polishing current: Diffusion layer ~ 0.1  Bath volume ~ 0.1  (Nb EP bath: 10 times less) Polishing Standard Cu Electropolishing: 55% vol. H 3 PO 4 45% vol. butanol

9 October 2006 Sergio Calatroni - CERN - Sputtering Technology10 Numerical modelling of the cathode by simulation of the entire EP process with the Elsy 2D/3D computer code ( Electropolishing – Cathode design Cathode active region Current density is uniform over all the cell surface

9 October 2006 Sergio Calatroni - CERN - Sputtering Technology11 Measurement of pinholes Irregularities on the substrate surface shadowing effect film inhomogeneities film p1p1 p2p2 Substrate disk Machining and cleaning Film deposition Substrate removal He leak rate experiment ‹  inc › fraction of leaky film surface - equator 9° 4.4 ppm - (~iris 50° 25 ppm) - equator 9° 0.1 ppm CP EP

9 October 2006 Sergio Calatroni - CERN - Sputtering Technology12 Defects in Cu substrate Electropolished copper surface average roughness: 0.02 µm A few defects still appearing Cross section of a copper cavity Defects are present inside ! Not an artifact of the preparation Thanks to: G. Arnau-Izquierdo

9 October 2006 Sergio Calatroni - CERN - Sputtering Technology13 Variation of properties with incidence angle Std.Dev. of grey levels of SEM images (CERN 1999) XRD spectra AFM roughness From: V. Palmieri, D. Tonini – INFN-LNL

9 October 2006 Sergio Calatroni - CERN - Sputtering Technology14 Incidence angle and residual resistance in low-  cavities Correlation between the incidence angle of the film and the residual resistance, measured on 352 MHz Nb/Cu cavities It seems there is a “threshold” effect

9 October 2006 Sergio Calatroni - CERN - Sputtering Technology15 Angle of incidence – post magnetron – conformal coating Niobium cathode Cavity B Magnetic field lines follow the cavity shape V. Palmieri, R. Preciso, V.L. Ruzinov, S. Yu. Stark, “A DC Post-magnetron configuration for niobium sputtering into 1.5 GHz Copper monocells”, Presented at the 7th Workshop on RF Superconductivity

9 October 2006 Sergio Calatroni - CERN - Sputtering Technology16 Angle of incidence in spherical cavity PACO cavity INFN Genova Standard 1.5 GHz cavity Average incidence angle of the Nb coating [degrees] Length along the cavity axis [mm] Average incidence angle of the Nb coating [degrees] The cathode is not point-like  The incidence angle is always >0

9 October 2006 Sergio Calatroni - CERN - Sputtering Technology17 Surface resistance of spherical cavity Rs [nOhm] Rs [Ohm] 1.7 K Standard 1.5 GHz Nb/Cu cavity Reducing the angle of incidence does not change R s (E). However, the angle is always greater than zero, and whether this is creating any effect is only matter of speculation – for the time being

9 October 2006 Sergio Calatroni - CERN - Sputtering Technology18 Thermal impedance film-substrate Cu (RRR=100) EP4300 ± 200 Wm -2 K -1 Cu EP µm Nb4100 ± 200 Wm -2 K -1 Nb (RRR=180) EP1200 ± 200 Wm -2 K -1 Nb EP µm Nb1000 ± 200 Wm -2 K -1 The overall thermal impedance has been measured for pure Nb and Cu, and for Nb/Cu and Nb/Nb films, on 2-mm thick disks. Nb (RRR=670)(extrap.)2500 ± 200 Wm -2 K -1 (Still lower than Nb/Cu, but Nb cavities performs better at high field !!) The thermal impedance of the film (if existing) has no effect on R res at high RF field Thanks to: G. Vandoni, J-M Rieubland, L. Dufay

9 October 2006 Sergio Calatroni - CERN - Sputtering Technology19 Nb/Nb 1p5.1 Nb bulk cavity 1p5.2 Nb/Nb (quench limited) Thanks to: V. Palmieri, D. Reschke, R. Losito

9 October 2006 Sergio Calatroni - CERN - Sputtering Technology20 Al substrate (better thermal conductivity) Al % purity: 10X thermal conductivity of Cu at 4.2 K Spinning + chemical polishing + coating Thanks to: V. Palmieri, G. Lanza

9 October 2006 Sergio Calatroni - CERN - Sputtering Technology21 Film structure – FIB cross sections Standard filmsOxide-free films 0.5 µm Courtesy: P. Jacob - EMPA Grain size with Focussed Ion Beam micrographs

9 October 2006 Sergio Calatroni - CERN - Sputtering Technology22 Grain size ~ 100 nm Fibre texture Diffraction pattern: powder diagram Grain size ~ 1-5 µm Heteroepitaxy Diffraction pattern: zone axis [110] 500nm TEM views I – plan view

9 October 2006 Sergio Calatroni - CERN - Sputtering Technology23 (110) fibre texture  substrate plane Heteroepitaxy Nb (110) //Cu(010) Nb (110) //Cu(111) Nb (100) //Cu(110) 500nm TEM views II – cross section

9 October 2006 Sergio Calatroni - CERN - Sputtering Technology24 200K 100K Crystallographic defects TEM views III - defects ~100 nm

9 October 2006 Sergio Calatroni - CERN - Sputtering Technology25 High-energy deposition techniques Crystalline defects, grains connectivity and grain size may be improved with an higher substrate temperature which provides higher surface mobility (important parameter is T substrate /T melting_of_film ) However the Cu substrate does not allow heating  The missing energy may be supplied by ion bombardment –In bias sputter deposition a third electron accelerates the noble gas ions, removing the most loosely bound atoms from the coating, while providing additional energy for higher surface mobility “Structure Zone Model” –Other techniques allow working without a noble gas, by ionising and accelerating directly the Nb that is going to make up the coating –These techniques allow also to obtain “conformal” coatings that follow the surface profile better filling voids.

9 October 2006 Sergio Calatroni - CERN - Sputtering Technology26 Nb/Cu bias deposition – First SEM images at CERN No biasBias -60 V Bias -80 V Bias -100 V 5 µm

9 October 2006 Sergio Calatroni - CERN - Sputtering Technology27 Biased sputtering at LNL

9 October 2006 Sergio Calatroni - CERN - Sputtering Technology28 Evaporation + ECR (JLAB) Niobium is evaporated by e-beam, then the Nb vapours are ionized by an ECR process. The Nb ions can be accelerated to the substrate by an appropriate bias. Energies in excess of 100 eV can be obtained. From: A.-M. Valente, G. Wu Generation of plasma inside the cavity 3 essential components: Neutral Nb vapor RF power 2.45GHz) Static B  ERF with ECR condition Why ECR? No working gas High vacuum means reduced impurities Controllable deposition energy, 90-degree deposition flux (Possible to help control the crystal structure) Excellent bonding No macro particles Faster rate (Conditional)

9 October 2006 Sergio Calatroni - CERN - Sputtering Technology29 Evaporation + ECR: results on samples Obvious advantage: no noble gas for plasma creation Sample tests: good RRR and Tc, 100-nm grain size, lower defect density and smooth surfaces 60eV 90eV 4000X4000 µm 2 3-D Profilometer ImagesTEM Images

9 October 2006 Sergio Calatroni - CERN - Sputtering Technology30 Application to cavities (JLAB)

9 October 2006 Sergio Calatroni - CERN - Sputtering Technology31 Plasma Arc (INFN) In the plasma arc an electric discharge is established directly onto the Nb target, producing a plasma plume from which ions are extracted and guided onto the substrate by a bias and/or magnetic guidance Magnetic filtering (and/or arc pulsing) is also necessary to remove droplets A trigger for the arc is necessary: either a third electrode, or a laser Arc spot moves on the Nb cathode at about 10 m/s Arc current is A Cathode voltage is ~ 35 V Ion current is mA on the sample-holder (2-10 mA/cm 2 ) Base vacuum ~ mbar Main gas during discharge is Hydrogen (~ mbar) Voltage bias on samples V From: R. Russo, A. Cianchi, S. Tazzari

9 October 2006 Sergio Calatroni - CERN - Sputtering Technology32 Plasma Arc – Need for filtering Arc source Nb droplets 5 µm 10 µm Magnetic filter Nb droplets

9 October 2006 Sergio Calatroni - CERN - Sputtering Technology33 Planar arc – cavity deposition set-up New ideas are put forward for using a planar arc for cavity coating 5 µm 10 µm

9 October 2006 Sergio Calatroni - CERN - Sputtering Technology34 Planar arc – RF measurements on samples! Cu samples with Nb ARC-coating. Used as a baseplate of 6 GHz cavity operating in the TE011 mode. At low field, the surface resistance is in the range 3÷6 µOhm, as compared to the BCS Rs of 0.22 µOhm at 2.2 K and small mean free path. The Q remained constant up to a field of 300 Oe. A baseline of 2.2 µOhm is measured with this cavity with a solid Nb plate

9 October 2006 Sergio Calatroni - CERN - Sputtering Technology35 Liner arc for cavity deposition (Soltan Institute)

9 October 2006 Sergio Calatroni - CERN - Sputtering Technology36 HPPMS: advantages From W. Sproul, AEI

9 October 2006 Sergio Calatroni - CERN - Sputtering Technology37 Principle: high power pulses of short duration –Peak value typically 100 times greater than conventional magnetron sputtering –Pulse width of µsec, repetition rate ~50 Hz –Peak power densities of 1-3 kW/cm 2 –Discharge voltages of V –Peak current densities ~ A/cm 2 Consequences –High degree of target material ionization –High secondary electron current –Promotes ionization of sputtered species –Can approach 100%, vs. ~1% for conventional sputtering –An applied bias allows attracting ions to the substrate Operating principle

9 October 2006 Sergio Calatroni - CERN - Sputtering Technology38 Ion density From W. Sproul, AEI

9 October 2006 Sergio Calatroni - CERN - Sputtering Technology39 HPPMS coatings in trenches (valid for any ion coating technique) J. Alami et Al, J.Vac.Sci.Technol. A23(2005)278

9 October 2006 Sergio Calatroni - CERN - Sputtering Technology40 First CERN results Pulse voltage (-500V/div) Pulse current (100A/div) Current on sample (-100 V bias, 1A/div)

9 October 2006 Sergio Calatroni - CERN - Sputtering Technology41 First SEM pictures (all films ~100 nm thick) Substrate floating Substrate grounded Substrate biased -100V Note: poor substrate preparation Unfortunately no new experiments were possible since last year

9 October 2006 Sergio Calatroni - CERN - Sputtering Technology42 Films as a bulk – Hydrogen becomes a problem! ~1 µm grain size, RRR  28 ~5 µm grain size, RRR  µm grain size, RRR  11 H 2 content is ~ 0.1 at. % for sputtered Nb/Cu films (in niobium bulk it is 0.02 at.%) and it is picked up from vacuum system during deposition. A possible solution: high-temperature annealing, but it does not work with copper cavities. Proposal (L. Hand, W. Frisken): molybdenum cavities. There are more differences between these Nb/Cu films than those listed, this is just a basis for reflection

9 October 2006 Sergio Calatroni - CERN - Sputtering Technology43 Film as a bulk – H 2 measurements From: L. Hand, Cornell U. – W. Frisken, York U. There are new results on measurements of H 2 content by measuring the lattice parameter and the total impurity content

9 October 2006 Sergio Calatroni - CERN - Sputtering Technology44 Grain boundaries and surface oxidation Famous drawings by Halbritter. Several effects might take place: ITE, flux penetration, H c1 depression, lower Tc, etc.

9 October 2006 Sergio Calatroni - CERN - Sputtering Technology45 Possible solution – Al 2 O 3 cap layers Technique routinely used for S-N-I-S Josephson junctions: a 5-nm thick Al layer is deposited onto the Nb base electrode, and let oxidize in air. Most of it is transformed to Al 2 O 3 but some remains metallic. It is important to prevent any surface contamination of Nb prior to Al coating, to reduce the coalescence of the Al atoms. Other possible solution: NbN overlayer (J. Halbritter) 5 nm Al (nominal)10 nm Al (nominal) XPS depth profile

9 October 2006 Sergio Calatroni - CERN - Sputtering Technology46 State-of-the-art 20 years ago

9 October 2006 Sergio Calatroni - CERN - Sputtering Technology47 State-of-the-art at 1500 MHz – 1.7 K – single cell Q  1x10 15 MV/m Q  3x10 20 MV/m 28 MV/m reached in a large LEP cryostat (He evaporation at large power)

9 October 2006 Sergio Calatroni - CERN - Sputtering Technology48 Conclusion In an ideal world niobium films would be the best solution for accelerating cavities at any beta. However they are presently not competitive for reaching the highest fields because of the increase of R res : WE MUST STRUGGLE AND UNDERSTAND WHY! Films are still a valuable option for lower fields and operation at 4.2 K The technique of choice is at present sputter deposition: a prerequisite for it is substrate design and its preparation Four proposed research lines: –Substrate effects –New deposition techniques (“energetic”) –Hydrogen effects –Cap layers Several of the above research lines are interdependent