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Field emission measurements on flat Cu samples relevant for CLIC accelerating strucutresS. Lagotzky, G. MllerUniversity of Wuppertal, FB C Physics Department, Wuppertal, Germany30-09-2014

Motivation and theoryMeasurement techniquesSamplesField emission resultsConclusions and OutlookAcknowledgements: Funding by BMBF project 05H12PX6Field emission measurements on flat Cu samples relevant for CLIC accelerating structuresStefan Lagotzky |# von 311Dark current / electric breakdown is the main field limitation of accelerating structures for CLIC (Eacc=100 MV/m, Epk=243 MV/m)Deep and quantitative understanding of the origin of breakdown processes is important!

Goal: Suppression of breakdowns by using proper surface treatmentsInvestigation of the enhanced field emission (EFE) from Cu surfaces as precursorof breakdownWhat causes EFE from (relevant) Cu samples?How to reduce/avoid EFE?MOTIVATION

Typical result of a dcdischarge on NbField emission measurements on flat Cu samples relevant for CLIC accelerating structuresStefan Lagotzky |# von 31Field emissionField Emission (FE): cold electron emission induced by high electric fields

Tunneling current through the resulting barrierRound 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/m2 @ 2000 MV/m

Field emission measurements on flat Cu samples relevant for CLIC accelerating structuresStefan Lagotzky |# von 31enhanced field Emission (EFE)Electric field enhancement factor:h = height of defect; r = curvature radiusEL = local electric field on defectES = electric field on flat surface

particles / protrusions:scratch:metal-insulator-metal (MIM):

emission area

activation by burning of conducting channels:MIVAmbientoxide layerNbInsulatorMIMNb(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 structuresStefan Lagotzky |# von 31Activation of emittersInsulating oxide layer (IOL, thickness dox ~ few nm) on metallic surfacesQuestion: How are emitters activated?

Surface defects (MIV)Particulates (MIM)Conducting channel (CC) is burned into oxide by activated emission currentalways strongerField emission measurements on flat Cu samples relevant for CLIC accelerating structuresStefan Lagotzky |# von 31Emitter number density N

*H. Padamsee et al., p. 998 1000, Proc. PAC1993.N0: normalization factorcs: surface condition factorNtot: total number of emittersField emission measurements on flat Cu samples relevant for CLIC accelerating structuresStefan Lagotzky |# von 31

Resulting N(Eact) dependenceN(Eact=0) = 0Exponential-like increase for low EactSee A. Navitski et al., PRSTAB 16, 112001 (2013)Saturation towards Ntot for high Eact Ntot,1>Ntot,2 and cs,1>cs,2

Field emission measurements on flat Cu samples relevant for CLIC accelerating structuresStefan Lagotzky |# von 31DC Field Emission Scanning Microscope (FESM)

sampleanodepiezotrans-latorselectron gunion gunRegulated voltage V(x,y) scans at fixed FE current (typ. I = 1nA) and gap z (anode = 300 m, scan range 2525 mm2, tilt correct. 1 m within 5 mm) emitter position, number density N and localization of emittersLocal U(z) & I(V) measurements of single emitters Eon(1 nA), FN, SFNIon gun (Eion = 0 5 keV), SEM (low res.), AES, heat treatments (< 1200C)Clean laminar air flow around load-lockEx-situ SEM & EDX: Identification of emitting defects (positioning accuracy ~100 m)

+AESField emission measurements on flat Cu samples relevant for CLIC accelerating structuresStefan Lagotzky |# von 31Surface quality controllOptical Profilometer (OP)white light irradiation and spectral reflection (chromatic aberration)20x20 cm scanning range in 2 cm distanceCurved surface up to 5 cm height difference2 m (3 nm) lateral (height) resolution Further zooming by AFM:2 m positioning relative to OP results98x98 m2 scanning range3 (1) nm lateral (height) resolutioncontact or non-contact modes.Clean laminar air flow (LAF) from the back Granite plate with active damping systemCCD camera for fast positioninginterferometric film thickness sensor (IF)

Field emission measurements on flat Cu samples relevant for CLIC accelerating structuresStefan Lagotzky |# von 31SAMPLES

Protection capSampleHolderFESM adapterInvestigatedsurfaceFlat Cu samples Diameter: ~11 mmHole as mark to relocate the emitter position in different systems (accuracy ~ 500 m)Diamond turned (DT) and partially chemically etched (0.6 m, SLAC treatment at RT for 5 s) using H3PO4 (70.0%), HNO3 (23.3%), acetic glacial acid (6.6%), and HCl (0.49%)Glued with SEM button on a holder and mounted to an adapter for the FESM at BUWFinal cleaning at BUWTeflon protection cap to avoid damage and contaminations after polishing and cleaningField emission measurements on flat Cu samples relevant for CLIC accelerating structuresStefan Lagotzky |# von 31SURFACE QUALITY

Sample surface very flat (0.5 m)Many pits (N < 18 mm-2) due to etchingGrain size: 1300 m - 5.3 mmAverage roughness: Ra/Rq = 150/230 nmSamples measured with OP before DIC in the FESM relevant area

DT & SLACSlightly waved surface (~ 0,5 1 mm)Many ridges from DTDamage layer?Average roughness: Ra/Rq = 126/145 nmDTField emission measurements on flat Cu samples relevant for CLIC accelerating structuresStefan Lagotzky |# von 31Field emission results DT + SLAC treatment

05 mm

5 mm0301505 mm

5 mm0603005 mm

5 mm010050Field [MV/m]Field [MV/m]Field [MV/m]Sample got DT and SLAC treatment, but no final cleaningFirst emissionat 30 MV/mN = 28 11 cm-2N = 152 25 cm-238 emission sites at Eact = 100 MV/mEmitter number density : 152 cm-2Emitter uniformly distributed in the scanned areaActivation field Eact > onset field EonEact = 80 MV/m, Eon =47 MV/mSimilar results on a second sample

Field emission measurements on flat Cu samples relevant for CLIC accelerating structuresStefan Lagotzky |# von 31Field emission results DT + ionized N2 cleaningSample got only DT and final cleaning by ionized N2 (p 5 bar)

First emissionat 130 MV/mN = 20 9 cm-2

1235N = 52 14 cm-213 emission sites at Eact = 190 MV/mEmitter number density : 52 cm-2Emitter uniformly distributed in the scanned areaActivation field Eact > onset field EonEact = 180 MV/m, Eon =114 MV/m

Field emission measurements on flat Cu samples relevant for CLIC accelerating structuresStefan Lagotzky |# von 3113EFE activation statisticsEmission from surfaces without any cleaning starts at 30 MV/mlg(N) increases linear with inverse field as expected229/372 emitters/cm2 on SLAC-etched samples without N2 at E = 243 MV/mCleaning with N2 shows a reduction of N down to 124 emitters/cm2 most probably due to removal of large particulatesLinear Fits A + BE-1 :A = 2.67119, B = -75.79643A = 2.87218, B = -73.42952A = 3.77774, B = -408.55786

Field emission measurements on flat Cu samples relevant for CLIC accelerating structuresStefan Lagotzky |# von 31Single emitter characteristics on SLAC-etched samples without cleaning

EDX shows S, Cl, KEDX shows S, Cl, SiEFE is dominated by particulates (d ~ 10 30 m) Removing particulates to reduce EFE

20 emitters investigated with SEM/EDX:12 particulates (Al, Cl, S, Si, K)2 surface defect6 emission sites: unknown origin

010050Field [MV/m]Field emission measurements on flat Cu samples relevant for CLIC accelerating structuresStefan Lagotzky |# von 31DRY ICE CLEANING SYSTEMCleaning the surface byPressure and shearing forces due to high velocity of snow crystalsBrittling of contaminations by rapid coolingPowerful rinsing due to the 500 times increased volume after sublimationSolvent cleaning by melted CO2 snow particlesParticulates with d > 100 nm are removedCommercial DIC system (SJ-10, CryoSnow) installed in cleanroom (class iso 5)

hand gunnozzleClean roomenvironmentcontrolpanelinlet CO2inlet N2Field emission measurements on flat Cu samples relevant for CLIC accelerating structuresStefan Lagotzky |# von 31Cleaning processCleaning of (grounded) samples with handgun (d ~ 5 cm) typically for 5 minLiquid CO2 (10 bar) and N2 (8 - 10 bar, propellant gas)Flat (12x3 mm) or round ( = 5 - 10 mm) jet of CO2 snow particlesSamples are treated 2.5 min under 90/ 45and 3 x rotated in 90stepsTeflon protection caps are cleaned as well

Field emission measurements on flat Cu samples relevant for CLIC accelerating structuresStefan Lagotzky |# von 31Avoiding particulate contaminationsA cleanroom environment (class ISO 3) was installed around the load-lock of the FESM to avoid particulate contaminations during installation of samplesProtection cap mechanically fixed until sample reaches cleanroom environmentProtection cap loosened under laminar air flowFinal removement of cap in preparation chamber at p ~ 10-7 mbar

Field emission measurements on flat Cu samples relevant for CLIC accelerating structuresStefan Lagotzky |# von 31EFE results after DIC on DT + SLAC sample (17E)Field maps between 120 - 300 MV/m, 20 (10) MV/m steps for E < (>) 200 MV/mScanned area: 5x5 mm, truncated cone anode (W, = 300 m), step size = 150 m, z = 25 m (E 240 MV/m), 40 m (200 240 MV/m) or 50 m (E < 200 MV/m)

No EFE at 120 MV/mDischarge at 140 MV/mFirst stable EFE at 240 MV/m14 emission sites (including discharge) at Eact = 300 MV/mEmitter number density : 56 cm-2EFE free region in the scanned area at E = 300 MV/mActivation field Eact > onset field EonEact = 260 MV/m, Eon =128 MV/m

Field emission measurements on flat Cu samples relevant for CLIC accelerating structuresStefan Lagotzky |# von 31EFE results after DIC on DT + SLAC sample (18E)Field maps between 140 - 260 MV/m, 10 (20) MV/m steps for E > ( onset field EonEact = 260 MV/m, Eon =168 MV/mField emission measurements on flat Cu samples relevant for CLIC accelerating structuresStefan Lagotzky |# von 31Efe activation statistics after dicDIC reduces N significantly from N= 229 cm-2 and N = 372 cm-2 (N = 124 cm-2) without (with) N2 cleaning to N=29 cm-2 @ E = 243 MV/m Chemical etching did not further reduce N log(N) increases nearly exponentially with E-1 as expectedBut still at least ~ 30 emitters in the iris area of a CLIC accelerating structure (~1 cm2)

Averaged over 4 samplesLinear Fits A + BE-1 :A = 3.77774, B = -408.5579A = 3.62966, B = -501.8538A = 2.95432, B = -362.2702Field emission measurements on flat Cu samples relevant for CLIC accelerating structuresStefan Lagotzky |# von 31SINGLE EMITTER CHARACTERISTICS on DIC samples Local I(V) curves of 49 emission sites selected from E(x,y) maps; SEM/EDX analysis of the Cu surface revealed: 57% surface defects; 12% small (< 2 m) particulates (Al, Si, W); 31% unidentifed;

Rather stable FN-like EFESlight jumps, probably due to melting of micro-tipsMore unstable EFEChanged slope at high fields due to bad electrical contact to bulkField emission measurements on flat Cu samples relevant for CLIC accelerating structuresStefan Lagotzky |# von 31Single emitter statistics

More examples for DT + SLAC + DIC samples:

CaSi, Al

Al, SiField emission measurements on flat Cu samples relevant for CLIC accelerating structuresStefan Lagotzky |# von 31Reproduction of dic-effect after slac etchingApplying DIC on DT sample has led to a significantly reduced N = 29/cm2

Field maps of two more DT+SLAC+DIC samples:

0

5 mm08040Field [MV/m]First emission(old results: 140 MV/m)

0

5 mm06040Field [MV/m]#1#2

0

5 mm016080Field [MV/m]0

5 mm016080Field [MV/m]

0

5 mm018090Field [MV/m]0

5 mm021080Field [MV/m]

Field emission measurements on flat Cu samples relevant for CLIC accelerating structuresStefan Lagotzky |# von 31Comparison to old results

New results on etched samples are worseA = 2.67119, B = -75.79643A = 2.87218, B = -73.42952N = 229 / 372 emitter /cm2 at E = 243 MV/mWhat is the reason for that?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 cavitiesField emission measurements on flat Cu samples relevant for CLIC accelerating structuresStefan Lagotzky |# von 31FN-PARAMETERS STATISTICSFitting the FN-equation to the FN-plots of every measured emitter using the assumption = 4.65 eV:

Characterizing every emitter by the resulting FN-parameters FN and SFN

A = 154, B = 6830[E] = MV/m, [I] = ASurface defects & particulates reveal mainly FN = 10 - 70 15% with FN < 150, mainly particulatesNo correlation with geometric field enhancement (SFN ~ FN-2), especially at low FN-valuesClear hint for other EFE mechanisms like MIV- and MIM-emission

Surface defectsParticulatesUnknownField emission measurements on flat Cu samples relevant for CLIC accelerating structuresStefan Lagotzky |# von 31Origin of breakdowns in rf structuresAfter activation: Field level of IFN = 1 nA is reduced Eon(1 nA) < EactDetermination of Eon(1 nA) by local emitter field calibration (U(z)-measurement)

Eon for surface defects & particulates similarFew emitters have very low Eon 3)90% of emitters with > 3 are caused by surface defects after DICHigh- 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 structuresStefan Lagotzky |# von 31Examples: Two candidates for breakdowns

Activated between 240 250 MV/m, Eon = 54 MV/m = 4.62 0.19Calculated current at 243 MV/m with FN = 17 and SFN = 1.11025 m! : IFN ~ 1022 A!

Activated between 130 140 MV/m, Eon = 80 MV/m = 1.75 0.12Calculated current at 243 MV/m with FN = 43 and SFN = 2.4102 m : IFN ~ 3 mANearly indipendent of

Field emission measurements on flat Cu samples relevant for CLIC accelerating structuresStefan Lagotzky |# von 31Observing discharges in the fesmSometimes discharges happen also during measurements in the FESM by accidentDuring scans if the current jump is faster than the voltage regulation (~2 ms)During local measurement because of activation effects

Discharges (most-likely caused by high- emitters) destroy the surface and lead to the formation of new stable and strong emitters, similar to BDs in cavitiesIn 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 BDRField emission measurements on flat Cu samples relevant for CLIC accelerating structuresStefan Lagotzky |# von 31conclusionsScaling law for field dependence of emitter number density on Cu samples foundActual surface quality of the Cu samples is not sufficient for high-gradient CLICstructures and shows up to N = 370 cm-2 at E = 243 MV/mDIC decreases N significantly by a factor of ~ 10 down to N = 29 cm-2 (E = 243 MV/m)Still not good enough for accelerating structuresMight reduce the BDR and/or the conditioning time of the accelerating structuresEtching the surfaces by SLAC treatment damages the surface and produces surface defects that emit at 243 MV/mReplacing SLAC treatment by electropolishing might reduce the BDR even moreGeometrical field enhancement is not sufficient to explain the observed EFE from relevant Cu surfaces Alternative emission processes like the MIV/MIM-model or voidsEmitters with > 2 are one candidate for causing BD in the accelerating structures and are mainly caused by remaining surface defects after DICField emission measurements on flat Cu samples relevant for CLIC accelerating structuresStefan Lagotzky |# von 31OUTLOOKImproving DIC by optimizing the parameters (pressure, cleaning time, cleaning procedure, CO2/N2 ratio, ) to remove even more particulatesMeasuring two samples with only DT after DICEmitter processing by current or by ion bombardment and SEM investigations before and after this conditioningField emission measurements on flat Cu samples relevant for CLIC accelerating structuresStefan Lagotzky |# von 31