Point contact tunneling spectroscopy and Atomic layer deposition for superconducting rf cavities Thomas Prolier, Mike Pellin, Jim Norem, Jeff Elam. Collaboration: -Jlab: P. Kneisel, G. Ciovati -Fermilab: L.Cooley, G. Wu, C. Cooper - IIT: J. Zasadzinki

Superconducting Radio-Frequency (SRF) Department of Energy – Office of Science DOE-OS is in the particle accelerator business (ILC ($19B), RIA($0.4 B), NSLS-2($0.5B), SNS($1.8B), APS, APS-ERL, etc.) Orbach to HEPAP 2/22/07 DOE is committed to continuing a vigorous R&D program of accelerator technology SCRF is a core capability having broad applicability, both to the ILC and to other future accelerator-based facilities as well. Out FY2008 request for ILC R&D and SCRF technology confirms this commitment 1 m 15 km 30 km of ultra pure Nb bellows 2 K very high electrical and magnetic fields

3 Outline Performance limitations: Point contact spectroscopy: a probe of the surface superconductivity. Atomic Layer Deposition: synthesizing new materials and application to RF cavities.

Niobium surfaces are complex, important, and currently poorly controlled at the nm level 4 45 nm RF depth Inclusions, Hydride precipitates Surface oxide Nb 2 O nm Magnetic! Interface: sub oxides NbO, NbO 2 often not crystalline (niobium-oxygen slush) Interstitials dissolved in niobium (mainly O, some C, N, H) Grain boundaries Residue from chemical processing Clean niobium e - flow only in the top 45 nm Probe the surface superconductivity

Point Contact Tunneling (PCT) Spectroscopy – a Surface Probe of Nb superconductivity 5 6 Tesla magnet K 2 Ideal BCS superconductor Measure of the superconducting gap Δ The ZBC value -> Number of normal electron Normal electrons in gap => dissipation and lower Q

PCT: 1- insight into Mild Baking Procedure Improvement: Small changes in O stoichiometry -> Magnetic Oxide reduction Unbaked Niobium T.Proslier, J.Zasadzinski, M.Pellin et al. APL 92, (2008) Cavity-grade niobium single crystal (110)-electropolished ILC-Single crystal cavities P.Kneisel Qo improvement 1.6 Average ZBC ratio = Ideal BCS, T=1.7K Baked Niobium 120C-24h Cavities have dissipative losses due to Cooper pair breaking! -> Nb 2 O 5-, NbO 2-

7 PCT: 2- Hot and cold spots in SRF cavity (from J-lab) Anomalous spectrum Only on hot spots Normal spectrum Hot spots: show dissipative behavior Higher ZBC and anomalous spec. lower gap values (1.3<<1.55) Cold spots: normal dissipation Low ZBC values Normal gap values (1.5<<1.55) Origin of peculiar spectrum and dissipation? Correlates with cavities results! (once again)

8 PCT: 2 Hot and cold spots in SRF cavity, Origin Temp. dep: peak at 0 mV bias increases Killing superconductivity by applying a mag. Field High bias peak: LOSSES!! fits with Appelbaum theory -> Magnetic impurities in the oxides !! J>0 -> antiferromagnetic coupling -First time measured on Nb oxides -Same behavior observed on unbaked Nb coupons ! IIT: EPR revealed magnetic moments too FSU: theory, dissipation and magnetism To be published

How to make better cavities Add a better dielectric (thanks to Intel) and bake O2O2 O

Atomic layer deposition (ALD) Al 2 O 3 (2nm) NbO x Nb Heating -> Reduction + diffusion of the oxides Baking, but now protected form O (Al 2 O 3 ) - (1.55meV = Nb). - Γ (dissipation) o C bake should significantly reduce dissipation Th.Proslier, J. Zasadzinski, M.Pellin et al. APL 93, (2008)

11 Cavities used for ALD Jlab (P. Kneisel) provided four different niobium cavities to ANL for atomic layer deposition: Cavity 1 : Material: RRR > 300 poly-crystalline Nb from Tokyo-Denkai Shape/frequency: Earlier KEK shape, 1300 MHz Baseline: electropolished, in-situ baked Cavity 2 : Material: RRR > 300 large grain Nb from Tokyo-Denkai Shape/frequency: TESLA/ILC shape, 1300 MHz Baseline: BCP, in – situ baked Cavity 3 : Material: RRR > 300 poly-crystalline Nb from Fansteel Shape/Frequency: CEBAF shape, 1497 MHz Baseline: BCP only Cavity 4: Shape/Frequency: CEBAF Single cell cavity Baseline: BCP o C UHV bake.

J Lab Cavity 1:After ALD Synthesis (10nm Al 2 O 3 + 3nm Nb 2 O 5 ), 250 o C Only last point shows detectable field emission. 2 nd test after 2 nd high pressure rinse. (1 st test showed field emission consistent with particulate contamination)

J lab Cavity 2: Large grain,10 nm Al2O3 + 3 nm Nb2O5 (250 o C) 13 Second coating: 5 nm Al 2 O nm Nb 2 O 5 First coating: 10 nm Al 2 O nm Nb 2 O 5 BaselineTest 2Test 1

J Lab Cavity 3: Annealing 450C/20hrs + Coating: 5nm Al 2 O nm Nb 2 O 5 14

High Temp. baking: T maps and Rs(T) T-map at the highest field measured during the test after 120 °C, 23 h UHV bake. T-map at the highest field measured during the test after 450 °C, 20 h heat treatment Treatment /kT c (nm) R res (n ) Add. HPR1.866 ± ± ± °C/23 h bake1.879 ± ± ± °C/20 h HT1.911 ± ± ± 0.2 Ohmic losses But HT baking: Improve the super. properties

Preliminary Conclusion and High temp annealing The ALD process is compatible with SRF cavity processing Promising if one thinks about multi-layer coatings ( A. Gurevich). development of the process is necessary The appearance of multipacting in cavity 1 and 2 is concerning, but can be overcome by additional coating. Baking doesnt improve cavity performance: cracks can appear due to strong Nb oxide reduction -> path for oxygen injection -> Ohmic losses need a in-situ baking + ALD coating set up. 16

17 ALD Can Produce Layered SRF Structures with significantly higher H c1 than Nb Build nanolaminates of superconducting materials ~ nm layer thicknesses with 10 nm Alumina Between. H c1 Enhancement Scales with: ~T c,laminate /T c,base For NbN laminate layers -> ~1.5 H C1 enhancement 50 MV/m -> 75 MV/m Nb, Pb Insulating layers Higher-T c SC: NbN, Nb 3 Sn, etc

SEM XPS XRR-RBS SQUID RRR New materials by Atomic Layer Deposition: NbF 5 + Si 2 H 6 = NbSi + reaction product, copy on : WF 6 + Si 2 H 6 = W + RP On Si (100): NbSi superconductor 3.1K On Quartz: Nb 3 Si 5 On MgO: NbSi 2 Elastic stress in the film-mismatch ? Nb 3 Si superconductor at 18K Vary substrate and growth conditions Post-annealing studies Model for A15 compounds: Nb 3 Ge (20K): NbF 5 + Ge 2 H 6 = NbGe + reaction products Etc… To be published Fast growth rate: 2.5 Å/Cy Grows only W Not on oxides

New material by Atomic Layer deposition New precursor NbF 5 for NbN, Nb 2 O 5 grows much faster! 19 Zinc pulse growth for NbN and TiN: NbCl 5 + NH 3 + Zn = NbN + ZnCl 2 + HCl TiN films: resistivity ρ=50 µΩ.cm for 10 nm films! (350 without Zn) NbN films: resistivity ρ=200 µΩ.cm (450 without Zn -> Tc= 5.5 K), same ρ for sputtered film with Tc=16K! To be measured: -Superconducting properties with Zn pulse -> Multilayers -Vary the substrate (Sapphires) to match lattice parameter (epitaxial growth?) -Post annealing in controlled atmosphere No studies of superconductivity by ALD and interactions substrate-films Phase space of parameter to study is large

Magnetic impurities as a possible explanation for RF dissipation: Mild baking effect Hot spots Origin = Oxides, vacancies? High temperature baking works on samples but not yet on cavities ALD a tool for building new materials Compatible with RF cavities NbN, NbSi, TiN etc… Plasma ALD Summary New task force: - Postdocs and students -> Accelerate the process

outlook 21 (1)Nb deposition on Nb a)New Cavity Designs b)Enable Continuity of Superconducting Surface (fewer perfect welds) (2)Other layered structures a)Reward : Performance far beyond NbN b)Risk: New ALD Synthesis Methods Need to be developed with semiconductor impurity levels. (3) Nb deposition on alumina coated Cu a)Reward : Significant Cost Reductions for Materials, Fabrication, and Cooling b)Risk: Dissimilar materials require stress management (Cu is bad, alumina is better) (4) Field emission for warm and cold cavities Particulate tolerant?

ALD Reaction Scheme ALD involves the use of a pair of reagents. each reacts with the surface completely each will not react with itself This setup eliminates line of site requirments Application of this AB Scheme Reforms the surface Adds precisely 1 monolayer Pulsed Valves allow atomic layer precision in growth Viscous flow (~1 torr) allows rapid growth ~1 m / 1-4 hours AB Cycles Thickness (Å) Ellipsometry Atomic Force Microscopy Film growth is linear with AB Cycles RMS Roughness = 4 Å (3000 Cycles) ALD Films Flat, Pinhole free

Mixed Oxide Deposition: Layer by Layer Mixed Layer Growth Layer by Layer note steps atomic layer sequence digitally controlled Films Have Tunable Resistivity, Refractive Index, Surface Roughness, etc. [(CH 3 ) 3 Al // H 2 O] 100 nm ZnO Al 2 O 3 [(CH 3 CH 2 ) 2 Zn // H 2 O] Mixed Layers w/ atomic precision Low Temperature Growth Transparent Uniform Even particles in pores can be coated.

Conformal Coating Removes Field Induced Breakdown Figure 3: Scanning Electron Microscope images of nearly atomically-sharp tips, before and after coating with a total of 35nm of material by ALD. The tip, initially about 4 nm, has been rounded to 35nm radius of curvature by growth of an ALD film. Rough surfaces are inherently smoothed by the process of conformal coating. Normal conducting systems ( cooling, CLIC ) can also benefit. ~100 nm smooth coatings should eliminate breakdown sites in NCRF. Copper is a hard material to deposit, and it may be necessary to study other materials and alloys. Some R&D is required. Nb ALD Coating Synthetic Development Needed Radius of Curvature of all asperities (when polishing is not enough) ALD can reduce field emission! Could allow separation of superconductor and cavity support materials (allowing increased thermal load, better mechanical stability)

ALD: The Only Viable Method for SRF Surface Control! 25 Niobium is from a surface scientists point of view a difficult material to deal with. –Extremely reactive. –Native Oxide is complex and passivates poorly Semiconductor Industry – a clue –Silicon is reactive but oxide is simple and passivates well (but has a low dielectric constant) –Gate dielectric oxides are now being used on Si metal (and being produced by ALD 20 m 2 / batch) Grow a dielectric oxide with superior properties to the Niobium Oxides –Simple - non-interactive with the sc layer –Passivating (stable surface, protective of the Nb metal underneath) –Parallel Growth Method Entirely adaptable to SRF Si HfO 2 Epoxy

ALD Thin Film Materials

A Solution? Atomic Layer Deposition -> non-dissipative dielectric layer 27 Mike Pellin 1.Use Atomic Layer Deposition (ALD) to synthesize a dielectric diffusion barrier on the Nb surface 2.Bake cavity to dissolve the O associated with the Nb layer into the bulk Nb NbO Nb 2 O 5- NbO 2 Al 2 O 3 Nb Al 2 O 3 ALD coated + Baking > 450°C Mild baked before ALD Test

Cavity 4: to be coated by SiN + NbSi (below 200 o C) 28 SRF 2009

Understanding Cavity E acc and Q 29 Q-slope problem Rs = R BCS + R res R BCS = C 4 2 l exp(- /kT) Experimental Goals: Measure at the surface Tunneling Spectroscopy is ideal P.Kneisel et al. 12 th SRF workshop Cornell 2005 B.Visentin SRF workshop 2003 G.Ciovati, P.Kneisel, A.Gurevich PRST, C.Antoine SRF workshop 2004 H( ) NbNbOxNbONb 2 O 5-δ NbO 2 B(r) Surface Q-slope disappears, Q 0 increased SRF Impedance is a surface effect ( ~45 nm, Nb) depends on the energy gap at the surface altered by proximity effects, magnetic scattering. SRF 2009

J-lab cavity 1 + HT annealing (450 o C for 20 hrs). 30 SRF 2009

J Lab Cavity 3: Small grain 2 steps Coating, first: 15 nm Al 2 O 3 at 90 o C 31 SRF 2009