1. Fachbereich C Physik UBW WORKSHOP, Berlin, 18. 12.2012 1 Activation and conditioning of field emitters on flat niobium surfaces Motivation and strategy Samples and measurement techniques Activation and conditioning by heat treatments Conditioning by ion bombardment Conclusion and outlook Stefan Lagotzky, Aliaksandr Navitski, Günter Müller Berg. Universität Wuppertal Detlef Reschke, Jörg Ziegler DESY Hamburg 18.12.2012

2. Fachbereich C Physik 2 Motivation and strategy o Enhanced field emission (EFE) from particulate contaminations/surface irregularities is one of the main field limitations of the high gradient superconducting Nb cavities required for XFEL (E acc =23.6MV/m, E pk /E acc =1.98) and ILC (E acc =31.5MV/m, E pk /E acc =2.4) o Systematic investigations of EFE has revealed 3 different activation mechanism: →Improved understandings of these processes are important 1.High fields 2.High temperatures 3.Discharges / microplasmas Strategy o Preparation of four flat Nb samples similar to the cavity fabrication process o Measuring of the initial EFE of the samples after the preparation o In-situ heating of the samples at different temperatures and measuring of the EFE after these heat treatments (HT) o In-situ ion bombardment of selected emitters and subsequent measurement of the EFE o Final High resolution SEM identification of the emitting defects UBW WORKSHOP, Berlin, 18. 12.2012

3. Fachbereich C Physik 3 Measurement techniques Field emission scanning microscope (FESM): o Regulated V(x,y) scans at fixed current I =1nA and gap ∆z  emitter position o Spatially resolved I(E) measurements of single emitters  E on, β E,FN, S o In-situ HT, ion bombardment (Ar, E ion = 0 - 5kV) and SEM (low res.) o In-situ heat treatments up to 1000°C 10 -9 mbar - localisation of emitters - FE properties 500 MV/m 10 -7 mbar Ex-situ SEM + EDXIdentification of emitting defects Correlation of surface features to FE properties (positioning accuracy ~ 100 µm) UBW WORKSHOP, Berlin, 18. 12.2012

4. Fachbereich C Physik 4 Samples Two single crystal (SC) and two large grain (LG) flat Nb-samples (RRR>250): o diameter of about 28 mm o contain 2 marks at the edges (90 0 ) for controlling the position in different measuring systems o 140µm Electropolishing (EP) at DESY o High pressure rinsing (HPR) of the surface at DESY (cleanroom class 10) o Transported under Teflon ® protection caps to avoid contaminations with particles/dust o The cap was not removed until the samples were under UHV conditions (~10 -7 mbar) Protection cap Measured surface Support rod UBW WORKSHOP, Berlin, 18. 12.2012

5. Fachbereich C Physik 5 Activation by high fields ~50MV/m Field [MV/m] o Exponential increase of the emitter density with increasing applied field o High activation field E acc necessary o Resulting lower onset field E on o Origin: particles, scratches, grain boundaries UBW WORKSHOP, Berlin, 18. 12.2012

6. Fachbereich C Physik 6 Heat Treatments (HT) o Heat treatments (HT) are a important part in the cavity fabrication process o Used for degassing hydrogen after polishing (HT at 800°C) o Also improvement of the superconducting properties (HT at 122°C) o Former results have shown that HT can produce emitters on a clean Nb surface Better understanding of this process is important E. Mahner, 1995: - increase of emitter density @ T < 800 0 C - decrease @ T < 1000 0 C - disappearing of emitters @ T > 1200 0 C - no clear correlation to surface defects A. Dangwal, 2007: evidence for grain boundary assisted FE @ 200 MV/m after 150 0 C for 14 h 800°C 120°C UBW WORKSHOP, Berlin, 18. 12.2012

7. Fachbereich C Physik 7 FE activation by HT o Recent results (2011): no emitter activation during HT at 122°C/24h, clear activation at HTs at 400°C and 800°C over 2h on 40µm BCP/HPR Nb o Origins: Foreign particles, scratches and other surface irregularities New series with more HTs between 122°C and 325°C and an additional HT at 400°C →Where does the activation start? →How are emitters influenced by the HT (conditioning)? HTHT122HT150HT175HT200HT250HT325 * HT400 T [°C]122150175200250325400 Duration [h]2412981062 Warm-up [h]3111111 initial 122°C/24h 400°C/2h 800°C/2h * Not on all samples UBW WORKSHOP, Berlin, 18. 12.2012

8. Fachbereich C Physik 8 BCP40 Field maps up to 160 MV/m before and after each HT of a LG sample A clear rise of the emitter density (N/cm²) with increasing temperature At low temperatures only one grain shows emitter activation, with increasing temperature the activation process also starts on the other grains Till the HT250 most emitters (60%) are activated near a grain boundary (HT400:11%) initial HT122 HT150HT175 HT200 HT250HT400 FESM results on one LG sample UBW WORKSHOP, Berlin, 18. 12.2012

9. Fachbereich C Physik 9 FESM results II o Number of by HT activated emitters rises with increasing temperature o Linear fit possible for T≤325°C o Emitter activation stronger for T>325°C o E on of HT activated emitters o E on of the emitters activated by HT400 is higher than of the other emitters o Two different types of emitters? o Two different activation processes? After 3 of 4 samples: UBW WORKSHOP, Berlin, 18. 12.2012

10. Fachbereich C Physik 10 → activation by burning of conducting channels: MIV Switch-on state persists! for a long period without E under UHV at RT → permanent formation of conducting channels [1] Ambient oxide layer Nb Insulator MIM Nb  ad- or desorption lead to enhancement or reduction of field  resonance tunneling can occur: [2] Impact for cavities:  Heating or rf power ? [4] activate emitters (1) Nb surface oxide: (3) Reduction of isolating Nb 2 O 5 layer by heat treatment [3]: (2) Adsorbates: [1] R. V. Latham, “High vacuum insulation,” (1995) [2] J. D. Jarvis, et al. IFEL 2010, WEPB46 (2010) [3] T. Proslier et.al. Appl. Phys. Let., 93, 192504 (2008) [4] J. W. Wang, et al. SLAC-PUB-7684 (1997) o Enhance MIM-activation of particles o Enhance MIV-activation or surface defects Nb 2 O 5 natural (~5nm) isolating NbO 2 semiconducting NbO metalic Possible explanations of the activation effect UBW WORKSHOP, Berlin, 18. 12.2012

11. Fachbereich C Physik 11 Dissolution of Oxygen in Nb during HT o During HTs the natural Nb 2 O 5 layer dissolves into NbO 2 and NbO Nb 2 O 5 NbO 2 NbO o Free oxygen is produces which is transported into the bulk o Amount of produced free oxygen: o Free oxygen can reach the bulk even at 122°C/24h since the diffusion length L ox is longer than the natural oxide thickness (5nm) o In this series the HTs are chosen so that C ox increases in one order of magnitude o This can be controlled by the temperature or by the duration UBW WORKSHOP, Berlin, 18. 12.2012

12. Fachbereich C Physik 12 o Conditioning of emitter 1 and emitter 3 by HT o Stability as well as E on is affected by the HTs o Two different emitters react in two completely different ways →HTs can make emitters either stronger (lower E on ) or weaker (higher E on ) →No clear trend can be observed →Is there a HT where most emitters become weak/strong? Conditioning by HT of single emitters FN-Plots (log(I/E²) vs 1/E)E on vs T #1 #3 UBW WORKSHOP, Berlin, 18. 12.2012

13. Fachbereich C Physik 13 Overview of FE properties InitialHT122HT150HT175HT200HT250HT400 #Β FN S FN Β FN S FN Β FN S FN Β FN S FN Β FN S FN Β FN S FN Β FN S FN 1681,0∙10 -5 771,5∙10 -5 811,4∙10 -4 711,1∙10 -3 789,9∙10 -5 731,6∙10 -2 615,2∙10 -4 2--751,2∙10 -6 562,2∙10 -6 423,8∙10 -5 313,9∙10 -2 504,3∙10 -5 716,8∙10 -6 3507,0∙10 -4 382,9∙10 -2 471,7∙10 -3 445,4∙10 -4 464,8∙10 -4 435,0∙10 -3 400,22 41001,3∙10 -5 621,1∙10 -2 643,2∙10 -2 1491,8∙10 -8 881,81013,2∙10 -5 771,1∙10 -2 5--278,6∙10 -2 222,9∙10 -1 265,8∙10 -2 2311,12181024667 6----211,9∙10 -1 192,4163,7∙10 5 259,9∙10 -2 142,8∙10 3 o Field enhancement factors β and emitting area S determined by FN plots o The highest and lowest values of β are marked o Every emitter reacts in a different way on the HTs o Impossible to identify a HT where most emitters are strong/weak →Any HT can produce a strong emitter! Local I-V measurements of one LG sample UBW WORKSHOP, Berlin, 18. 12.2012

14. Fachbereich C Physik 14 Ion bombardment Primary emitter FE current Ion pressure Cathode plasma Formation of new emitters o Micro discharges can produce ions above a primary emitter o Ions are accelerated towards the surface o Primary emitter can be destroyed, but new emitters can be formed o Question: How does an emitter change after a (controlled) ion bombardment with Ar? o Deactivation of strong emitter? (Conditioning/ processing) UBW WORKSHOP, Berlin, 18. 12.2012

15. Fachbereich C Physik 15 First results on ion bombardment 1 2 3 1.Emitter 1 & A: E ion =1kV, t=1h 2.Emitter 2 and B: E ion 1kV, t=1h 3.Emitter 2 and B: E ion =2kV, t=1h All emitters were activated at HT250 or below o After HTs and before ion bombardement the sample was exposed to air (clean room) o Deactivation of emitter A o Forming two new emitters o Emitters got basically weaker o Even at moderate ion energy (max. 5kV) initialSputter 1Sputter 2Sputter 3 #βSβSβSβS 11146∙10 -6 851∙10 -4 851∙10 -4 851∙10 -4 2584∙10 -6 584∙10 -6 47332570 B100 574∙10 -2 693∙10 -7 discharge UBW WORKSHOP, Berlin, 18. 12.2012

16. Fachbereich C Physik 16 Correlated SEM images of emitter activated at HT250 or below o SEM images taken after the HTs and after the ion bombardment o Only surface defects were activated at HT250 or below o Observed weakening of the emitter due to ion bombardment because of removal of sharp tips? Emitter B Emitter 1 UBW WORKSHOP, Berlin, 18. 12.2012

17. Fachbereich C Physik 17 o SEM images taken after the HTs and after the ion bombardment o Particles and leftover of particles were found o Reason of the different onset fields? o Such emitters were deactivated after exposed to air (clean room) Before clean roomafter clean room Correlated SEM images of emitter activated at HT400 UBW WORKSHOP, Berlin, 18. 12.2012

18. Fachbereich C Physik 18 Conclusions and outlook Conclusions: o Heating of clean, flat Nb samples leads definitely to activation of field emitters o Number density of emitters increase exponentially with electric field and linearly with temperature (T≤325°C) o Nb surface oxide seems to be responsible for the activation o Activation of surface defects sets in at lower temperatures than activation of particles o The activation on one grain sets in earlier than on the others → depending on the grain orientation? o Already activated emitters can either become stronger or weaker during a HT o Ion bombardment can make emitters basically weaker or remove them completely even at low ion energy, but also can lead to new emitters Outlook o Remaining sample will be measured to improve the statistics o More SEM images will be taken after the HT, before and after the ion bombardment to find out what exactly happens during the sputtering o Sputtering at higher ion energies and/or different sputtering times Acknowledgements D. Reschke and J. Ziegler at DESY for HPR, A. Matheisen and N. Steinhau-Kuehl for EP; funding by HGF Alliance and the BMBF project 05H09PX5. UBW WORKSHOP, Berlin, 18. 12.2012

19. Fachbereich C Physik 19 Surface roughness: Electric field enhancement factor: h = height of defect r = curvature radius E L = local electric field on defect E S = electric field on flat surface z(x i, y j ) = actual height value of profile n, m = No. of points in x and y direction = average height value Field enhancement particles / protrusions:Scratch / erosion:metal-insulator-metal (MIM): → emission area UBW WORKSHOP, Berlin, 18. 12.2012