Field emission measurements on flat Cu samples relevant for CLIC accelerating structures Stefan Lagotzky | 1 von 31 Field emission measurements on flat Cu samples relevant for CLIC accelerating strucutres S. Lagotzky, G. Müller University of Wuppertal, FB C – Physics Department, Wuppertal, Germany Motivation and theory 2.Measurement techniques 3.Samples 4.Field emission results 5.Conclusions and Outlook 1.Motivation and theory 2.Measurement techniques 3.Samples 4.Field emission results 5.Conclusions and Outlook Acknowledgements: Acknowledgements: Funding by BMBF project 05H12PX6

Field emission measurements on flat Cu samples relevant for CLIC accelerating structures Stefan Lagotzky | 2 von 31 o Dark current / electric breakdown is the main field limitation of accelerating structures for CLIC (E acc =100 MV/m, E pk =243 MV/m) o Deep and quantitative understanding of the origin of breakdown processes is important! o Goal: Suppression of breakdowns by using proper surface treatments →Investigation of the enhanced field emission (EFE) from Cu surfaces as precursor of breakdown What causes EFE from (relevant) Cu samples? How to reduce/avoid EFE? MOTIVATION Typical result of a dc discharge on Nb

Field emission measurements on flat Cu samples relevant for CLIC accelerating structures Stefan Lagotzky | 3 von 31 FIELD EMISSION Field Emission (FE): „cold“ electron emission induced by high electric fields o Tunneling current through the resulting barrier o Round shape of the barrier caused by the mirror charge (100MV/m: 0.38eV) vs. dc FE current given by Fowler-Nordheim (FN) results in a straight line (FN-Plot) For Nb: 1nA/μm 2000 MV/m

Field emission measurements on flat Cu samples relevant for CLIC accelerating structures Stefan Lagotzky | 4 von 31 ENHANCED FIELD EMISSION (EFE) 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 particles / protrusions: scratch: metal-insulator-metal (MIM): → emission area → activation by burning of conducting channels: MIV Ambient oxide layer Nb Insulator MIM Nb (1) Nb surface oxide:  ad- or desorption lead to enhancement or reduction of field  resonance tunneling can occur: (2) Adsorbates:

Field emission measurements on flat Cu samples relevant for CLIC accelerating structures Stefan Lagotzky | 5 von 31 ACTIVATION OF EMITTERS o Insulating oxide layer (IOL, thickness d ox ~ few nm) on metallic surfaces o Question: How are emitters activated? Surface defects (MIV) Particulates (MIM) Conducting channel (CC) is burned into oxide by activated emission current always stronger

Field emission measurements on flat Cu samples relevant for CLIC accelerating structures Stefan Lagotzky | 6 von 31 EMITTER NUMBER DENSITY N *H. Padamsee et al., p. 998 – 1000, Proc. PAC1993. N 0 : normalization factor c s : surface condition factor N tot : total number of emitters

Field emission measurements on flat Cu samples relevant for CLIC accelerating structures Stefan Lagotzky | 7 von 31 Resulting N(E act ) dependence 1.N(E act =0) = 0 2.Exponential-like increase for low E act See A. Navitski et al., PRSTAB 16, (2013) 3.Saturation towards N tot for high E act N tot,1 >N tot,2 and c s,1 >c s,2

Field emission measurements on flat Cu samples relevant for CLIC accelerating structures Stefan Lagotzky | 8 von 31 DC FIELD EMISSION SCANNING MICROSCOPE (FESM) sample anode piezotrans- lators electron gun ion gun o Regulated voltage V(x,y) scans at fixed FE current (typ. I = 1nA) and gap ∆z (Ø anode = 300 µm, scan range ≤ 25 ˣ 25 mm 2, tilt correct. ±1 µm within ±5 mm) → emitter position, number density N and localization of emitters o Local U(z) & I(V) measurements of single emitters → E on (1 nA), β FN, S FN o Ion gun (E ion = 0 – 5 keV), SEM (low res.), AES, heat treatments (< 1200°C) o Clean laminar air flow around load-lock o Ex-situ SEM & EDX: Identification of emitting defects (positioning accuracy ~100 µm) +AES

Field emission measurements on flat Cu samples relevant for CLIC accelerating structures Stefan Lagotzky | 9 von 31 SURFACE QUALITY CONTROLL o Optical Profilometer (OP) white light irradiation and spectral reflection (chromatic aberration) 20x20 cm² scanning range in 2 cm distance Curved surface up to 5 cm height difference 2 µm (3 nm) lateral (height) resolution o Further zooming by AFM: ±2 µm positioning relative to OP results 98x98 µm 2 scanning range 3 (1) nm lateral (height) resolution contact or non-contact modes. o Clean laminar air flow (LAF) from the back o Granite plate with active damping system o CCD camera for fast positioning o interferometric film thickness sensor (IF)

Field emission measurements on flat Cu samples relevant for CLIC accelerating structures Stefan Lagotzky | 10 von 31 SAMPLES Protection cap Sample Holder FESM adapter Investigated surface o Flat Cu samples o Diameter: ~11 mm o Hole as mark to relocate the emitter position in different systems (accuracy ~ 500 µm) o Diamond turned (DT) and partially chemically etched (0.6 µm, SLAC treatment at RT for 5 s) using H 3 PO 4 (70.0%), HNO 3 (23.3%), acetic glacial acid (6.6%), and HCl (0.49%) o Glued with SEM button on a holder and mounted to an adapter for the FESM at BUW o Final cleaning at BUW o Teflon protection cap to avoid damage and contaminations after polishing and cleaning

Field emission measurements on flat Cu samples relevant for CLIC accelerating structures Stefan Lagotzky | 11 von 31 SURFACE QUALITY o Sample surface very flat (±0.5 µm) o Many pits (N < 18 mm -2 ) due to etching o Grain size: 1300 µm² mm² o Average roughness: R a /R q = 150/230 nm Samples measured with OP before DIC in the FESM relevant area DT & SLAC o Slightly waved surface (λ~ 0,5 – 1 mm) o Many ridges from DT o Damage layer? o Average roughness: R a /R q = 126/145 nm DT

Field emission measurements on flat Cu samples relevant for CLIC accelerating structures Stefan Lagotzky | 12 von 31 FIELD EMISSION RESULTS DT + SLAC TREATMENT 05 mm mm mm Field [MV/m] Sample got DT and SLAC treatment, but no final cleaning First emission at 30 MV/m N = 28 ± 11 cm -2 N = 152 ± 25 cm -2 o 38 emission sites at E act = 100 MV/m →Emitter number density : 152 cm -2 o Emitter uniformly distributed in the scanned area o Activation field E act > onset field E on E act = 80 MV/m, E on =47 MV/m Similar results on a second sample

Field emission measurements on flat Cu samples relevant for CLIC accelerating structures Stefan Lagotzky | 13 von 31 FIELD EMISSION RESULTS DT + IONIZED N 2 CLEANING Sample got only DT and final cleaning by ionized N 2 (p ≈ 5 bar) First emission at 130 MV/m N = 20 ± 9 cm N = 52 ± 14 cm -2 o 13 emission sites at E act = 190 MV/m →Emitter number density : 52 cm -2 o Emitter uniformly distributed in the scanned area o Activation field E act > onset field E on E act = 180 MV/m, E on =114 MV/m

Field emission measurements on flat Cu samples relevant for CLIC accelerating structures Stefan Lagotzky | 14 von 31 EFE ACTIVATION STATISTICS o Emission from surfaces without any cleaning starts at 30 MV/m o lg(N) increases linear with inverse field as expected o 229/372 emitters/cm 2 on SLAC-etched samples without N 2 at E = 243 MV/m o Cleaning with N 2 shows a reduction of N down to 124 emitters/cm 2 most probably due to removal of large particulates Linear Fits A + B×E -1 : o A = , B = o A = , B = o A = , B =

Field emission measurements on flat Cu samples relevant for CLIC accelerating structures Stefan Lagotzky | 15 von 31 SINGLE EMITTER CHARACTERISTICS ON SLAC-ETCHED SAMPLES WITHOUT CLEANING EDX shows S, Cl, K EDX shows S, Cl, Si EFE is dominated by particulates (d ~ 10 – 30 µm) → Removing particulates to reduce EFE 20 emitters investigated with SEM/EDX: o 12 particulates (Al, Cl, S, Si, K) o 2 surface defect o 6 emission sites: unknown origin Field [MV/m]

Field emission measurements on flat Cu samples relevant for CLIC accelerating structures Stefan Lagotzky | 16 von 31 DRY ICE CLEANING SYSTEM o Cleaning the surface by  Pressure and shearing forces due to high velocity of snow crystals  Brittling of contaminations by rapid cooling  Powerful rinsing due to the 500 times increased volume after sublimation  Solvent cleaning by melted CO 2 snow particles o Particulates with d > 100 nm are removed o Commercial DIC system (SJ-10, CryoSnow) installed in cleanroom (class iso 5) hand gun nozzle Clean room environment control panel inlet CO 2 inlet N 2

Field emission measurements on flat Cu samples relevant for CLIC accelerating structures Stefan Lagotzky | 17 von 31 CLEANING PROCESS o Cleaning of (grounded) samples with handgun (d ~ 5 cm) typically for 5 min o Liquid CO 2 (10 bar) and N 2 ( bar, propellant gas) →Flat (12x3 mm) or round (Ø = mm) jet of CO 2 snow particles o Samples are treated 2.5 min under 90°/ 45°and 3 x rotated in 90°steps o Teflon protection caps are cleaned as well

Field emission measurements on flat Cu samples relevant for CLIC accelerating structures Stefan Lagotzky | 18 von 31 AVOIDING PARTICULATE CONTAMINATIONS o A cleanroom environment (class ISO 3) was installed around the load-lock of the FESM to avoid particulate contaminations during installation of samples o Protection cap mechanically fixed until sample reaches cleanroom environment o Protection cap loosened under laminar air flow o Final removement of cap in preparation chamber at p ~ mbar

Field emission measurements on flat Cu samples relevant for CLIC accelerating structures Stefan Lagotzky | 19 von 31 EFE RESULTS AFTER DIC ON DT + SLAC SAMPLE (17E) o Field maps between MV/m, 20 (10) MV/m steps for E ) 200 MV/m o Scanned area: 5x5 mm², truncated cone anode (W, Ø= 300 µm), step size = 150 µm, Δz = 25 µm (E ≥ 240 MV/m), 40 µm (200 – 240 MV/m) or 50 µm (E < 200 MV/m) No EFE at 120 MV/mDischarge at 140 MV/m First stable EFE at 240 MV/m o 14 emission sites (including discharge) at E act = 300 MV/m →Emitter number density : 56 cm -2 o EFE free region in the scanned area at E = 300 MV/m o Activation field E act > onset field E on E act = 260 MV/m, E on =128 MV/m

Field emission measurements on flat Cu samples relevant for CLIC accelerating structures Stefan Lagotzky | 20 von 31 EFE RESULTS AFTER DIC ON DT + SLAC SAMPLE (18E) o Field maps between MV/m, 10 (20) MV/m steps for E > (<) 200 MV/m o Scanned area: 5x5 mm², truncated cone anode (W, Ø= 300 µm), step size = 150 µm, Δz = 25 µm (E ≥ 240 MV/m), 40 µm (180 – 240 MV/m) or 50 µm (E < 180 MV/m) o 23 emission sites at E act = 260 MV/m →Emitter number density : 92 cm -2 o EFE free region in the scanned area at E = 260 MV/m o Activation field E act > onset field E on E act = 260 MV/m, E on =168 MV/m

Field emission measurements on flat Cu samples relevant for CLIC accelerating structures Stefan Lagotzky | 21 von 31 EFE ACTIVATION STATISTICS AFTER DIC o DIC reduces N significantly from N= 229 cm -2 and N = 372 cm -2 (N = 124 cm -2 ) without (with) N 2 cleaning to N=29 cm E = 243 MV/m o Chemical etching did not further reduce N o log(N) increases nearly exponentially with E -1 as expected o But still at least ~ 30 emitters in the iris area of a CLIC accelerating structure (~1 cm 2 ) Averaged over 4 samples Linear Fits A + B×E -1 : o A = , B = o A = , B = o A = , B =

Field emission measurements on flat Cu samples relevant for CLIC accelerating structures Stefan Lagotzky | 22 von 31 SINGLE EMITTER CHARACTERISTICS ON DIC SAMPLES o Local I(V) curves of 49 emission sites selected from E(x,y) maps; o SEM/EDX analysis of the Cu surface revealed: 57% surface defects; 12% small (< 2 µm) particulates (Al, Si, W); 31% unidentifed; o Rather stable FN-like EFE o Slight jumps, probably due to melting of micro-tips o More unstable EFE o Changed slope at high fields due to bad electrical contact to bulk

Field emission measurements on flat Cu samples relevant for CLIC accelerating structures Stefan Lagotzky | 23 von 31 SINGLE EMITTER STATISTICS More examples for DT + SLAC + DIC samples: Ca Si, Al Al, Si

Field emission measurements on flat Cu samples relevant for CLIC accelerating structures Stefan Lagotzky | 24 von 31 REPRODUCTION OF DIC-EFFECT AFTER SLAC ETCHING o Applying DIC on DT sample has led to a significantly reduced N = 29/cm 2 Field maps of two more DT+SLAC+DIC samples: 0 5 mm Field [MV/m] First emission (old results: 140 MV/m) 0 5 mm Field [MV/m] #1 #2 0 5 mm Field [MV/m] 0 5 mm Field [MV/m] 0 5 mm Field [MV/m] 0 5 mm Field [MV/m]

Field emission measurements on flat Cu samples relevant for CLIC accelerating structures Stefan Lagotzky | 25 von 31 COMPARISON TO OLD RESULTS o New results on etched samples are worse o A = , B = o A = , B = o N = 229 / 372 emitter /cm 2 at E = 243 MV/m What is the reason for that? o Only 1 particulate on two samples but 7 surface defects (etched pits + stains) most-likely caused by SLAC treatment! Etching damages the surface and is too bad for high-gradient cavities

Field emission measurements on flat Cu samples relevant for CLIC accelerating structures Stefan Lagotzky | 26 von 31 FN-PARAMETERS STATISTICS o Fitting the FN-equation to the FN-plots of every measured emitter using the assumption φ = 4.65 eV: →Characterizing every emitter by the resulting FN-parameters β FN and S FN A = 154, B = 6830 [E] = MV/m, [I] = A o Surface defects & particulates reveal mainly β FN = o 15% with β FN < 150, mainly particulates o No correlation with geometric field enhancement (S FN ~ β FN -2 ), especially at low β FN -values →Clear hint for other EFE mechanisms like MIV- and MIM-emission Surface defects Particulates Unknown

Field emission measurements on flat Cu samples relevant for CLIC accelerating structures Stefan Lagotzky | 27 von 31 ORIGIN OF BREAKDOWNS IN RF STRUCTURES o After activation: Field level of I FN = 1 nA is reduced → E on (1 nA) < E act o Determination of E on (1 nA) by local emitter field calibration (U(z)-measurement) o E on for surface defects & particulates similar o Few emitters have very low E on <100 MV/m! o Measure for the activation strength: field reduction factor ρ = E act /E on o Determination of ρ for 49 emission sites:  61% (20%) of emitters show ρ 3)  90% of emitters with ρ > 3 are caused by surface defects after DIC High-ρ emitters are most likely candidates for triggering BDs in accelerating structures due to the exponential current increase after their activation!

Field emission measurements on flat Cu samples relevant for CLIC accelerating structures Stefan Lagotzky | 28 von 31 EXAMPLES: TWO CANDIDATES FOR BREAKDOWNS o Activated between 240 – 250 MV/m, E on = 54 MV/m → ρ = 4.62 ± 0.19 →Calculated current at 243 MV/m with β FN = 17 and S FN = 1.1×10 25 m²! : I FN ~ A! o Activated between 130 – 140 MV/m, E on = 80 MV/m → ρ = 1.75 ± 0.12 →Calculated current at 243 MV/m with β FN = 43 and S FN = 2.4×10 2 µm² : I FN ~ 3 mA Nearly indipendent of φ

Field emission measurements on flat Cu samples relevant for CLIC accelerating structures Stefan Lagotzky | 29 von 31 OBSERVING DISCHARGES IN THE FESM Sometimes discharges happen also during measurements in the FESM by accident  During scans if the current jump is faster than the voltage regulation (~2 ms)  During local measurement because of activation effects o Discharges (most-likely caused by high-ρ emitters) destroy the surface and lead to the formation of new stable and strong emitters, similar to BDs in cavities o In accelerating structures this new emitter triggers the next BD, which forms another emitter, that ignite a BD etc. Avoiding the original emitter potentially avoids the following BDs and reduces the BDR

Field emission measurements on flat Cu samples relevant for CLIC accelerating structures Stefan Lagotzky | 30 von 31 CONCLUSIONS o Scaling law for field dependence of emitter number density on Cu samples found o Actual surface quality of the Cu samples is not sufficient for high-gradient CLIC structures and shows up to N = 370 cm -2 at E = 243 MV/m o DIC decreases N significantly by a factor of ~ 10 down to N = 29 cm -2 (E = 243 MV/m)  Still not good enough for accelerating structures  Might reduce the BDR and/or the conditioning time of the accelerating structures o Etching the surfaces by SLAC treatment damages the surface and produces surface defects that emit at 243 MV/m →Replacing SLAC treatment by electropolishing might reduce the BDR even more o Geometrical field enhancement is not sufficient to explain the observed EFE from relevant Cu surfaces → Alternative emission processes like the MIV/MIM-model or voids o Emitters with ρ > 2 are one candidate for causing BD in the accelerating structures and are mainly caused by remaining surface defects after DIC

Field emission measurements on flat Cu samples relevant for CLIC accelerating structures Stefan Lagotzky | 31 von 31 OUTLOOK o Improving DIC by optimizing the parameters (pressure, cleaning time, cleaning procedure, CO 2 /N 2 ratio, …) to remove even more particulates o Measuring two samples with only DT after DIC o Emitter processing by current or by ion bombardment and SEM investigations before and after this conditioning