1. Impact of dry ice cleaning on the enhanced field emission from flat Cu samples
S. Lagotzky, G. Müller
University of Wuppertal, FB C – Physics Department, Wuppertal, Germany
T. Muranaka, S. Calatroni
CERN, Geneva, Switzerland
Motivation and strategy
Measurement techniques
Samples
Field emission properties
Conclusions and Outlook
Acknowledgements: Funding by BMBF project 05H12PX6
4th International Workshop on Mechanisms of Vacuum Arcs (MeVArc 2013)

2. MOTIVATION
Dark 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 origins of breakdown processes is important!
Goal: Suppressing breakdowns by using proper surface treatments
Investigation of the enhanced field emission (EFE) from Cu surfaces as the precursor process of a breakdown
What causes EFE from (relevant) Cu samples?
How to avoid EFE?
Typicall result of a dc discharge on Nb

3. DC Field Emission Scanning Microscope (FESM)
ion gun
electron gun
anode
sample
piezotrans-
lators
Regulated voltage V(x,y) scans at fixed FE current (typ. I = 1nA) and gap ∆z (Øanode = 300 µm, scan range ≤ 25ˣ25 mm2, tilt correct. ±1 µm within ±5 mm)
emitter position, number density N and localization of emitters
Local U(z) & I(V) measurements of single emitters → Eon(1 nA), βFN, SFN
Ion bombardement (Eion = 0 – 5 keV), SEM (low res.), heat treatments (< 1200°C)
Ex-situ SEM & EDX: Identification of emitting defects (positioning accuracy ~100 µm)

4. Surface quality controll
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
Further zooming by AFM:
±2 µm positioning relative to OP results
98x98 µm2 scanning range
3 (1) nm lateral (height) resolution
contact or non-contact modes.
Clean laminar air flow (LAF) from the back
Granite plate with active damping system
CCD camera for fast positioning
interferometric film thickness sensor (IF)

5. DRY ICE CLEANING SYSTEM
Commercial DIC system (SJ-10, CryoSnow)
Installed in cleanroom (class iso 5) at BUW
Liquid CO2 (10 bar) and N2 (8 bar, propellant gas)
Flat (12x3 mm) or round (Ø = 10 mm) jet of CO2 snow particles
Cleaning of (grounded) samples with handgun (d ~ 5 cm) typically for 5 min
Samples are treated 2.5 min under 90°/ 45°and 3 x rotated in 90°steps
inlet CO2
Clean room
environment
inlet N2
control
panel
hand gun
nozzle

6. SAMPLES Two flat Cu samples (17E & 18E) Diameter: ~11 mm
One hole as mark to identify the position in different systems
Diamond turned and chemically etched (0.6 µm, SLAC treatment)
SLAC treatment: etching with H3PO4 (70.0%), HNO3 (23.3%), acetic glacial acid (6.6%), and HCl (0.49%) at RT for 5 seconds
Glued on a holder and mounted to an adapter for the FESM at BUW
Surface cleaned with DIC (5 min.)
Teflon protection cap (DIC for 3 min.) to avoid damage and contaminations after polishing and cleaning
Protection cap
Investigated
surface
Sample
Holder
FESM adapter

7. SURFACE QUALITY FESM relevant area (topography)
Samples measured with OP before DIC
Intensity of reflected light
Sample surface very flat (±0.5 µm)
Many pits (N < 18 mm-2)
Grain size: 1300 µm² mm²
Average roughness:
Ra = 150 nm & Rq = 230 nm

8. TYPICAL SURFACE DEFECTS
Grain boundaries (Δz = 500 – 800 nm)
Scratch (l > 1 mm, Δz < 1 µm)
Pit (Ø < 50 µm, Δz < 2 µm, N < 18 mm-2)
Particulate
Ridges
Curvature radius of 1, 3 & 4 may be < 2 µm
AFM measurements will be done later
More accurate estimations of βgeo possible
Comparing to a diamond turned surface (without etching, presented at MeVArc 2012)
Ridges are nearly removed after etching
Pits are caused by the etching
Enhanced etching rate, e.g. at dislocations
Roughness not significantly decreased:
Ra = 158 nm & Rq = 204 nm

9. EMITTER NUMBER DENSITY (n) AT DIFFERENT FIELD LEVELS on sample 17E
Field maps between MV/m, 20 (10) MV/m steps for E < (>) 200 MV/m
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/m
Discharge at 140 MV/m
First stable EFE at 240 MV/m
14 emission sites (including discharge) at Eact = 300 MV/m
Emitter number density : 56 cm-2
EFE free region in the scanned area at E = 300 MV/m
Activation field Eact > onset field Eon
Eact = 260 MV/m, Eon =128 MV/m

10. EMITTER NUMBER DENSITY (n) AT DIFFERENT FIELD LEVELS on sample 18E
Field maps between MV/m, 10 (20) MV/m steps for E > (<) 200 MV/m
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)
23 emission sites at Eact = 260 MV/m
Emitter number density : 92 cm-2
EFE free region in the scanned area at E = 260 MV/m
Activation field Eact > onset field Eon
Eact = 260 MV/m, Eon =168 MV/m

11. EFE statistics
N(Eact) for diamond turned Cu, same sample after DIC and for samples 17E & 18E
Impact of DIC clearly visible → reduced N (factor Eact = 200 MV/m
N(Eact) increases exponentially as for Nb and Mo → similar activation process
Similar slope for diamond turned + DIC sample and sample 18E
Best result for 17E, but slope modified by discharge (N offset)
Epeak (CLIC) = 243 MV/m
At 243 MV/m N ~ 394 / 42 / 20 cm-2 for diamond turned / diamond turned + DIC / diamond turned + etching + DIC Cu
Area of a single iris with E > 0.8×Epeak: ~100 mm²
Surface quality still not good enough for CLIC structures!

12. SINGLE EMITTER CHARACTERISTICS
Measuring I(E)-curves and making SEM/EDX investigations correlated to the field maps
Two Examples from 17E:
β~20, S ~10-3 µm²
β~13, S ~10-1 µm²
Al, Si

13. Single emitter statistics
Characterization of 25 emitters on sample 17E & 18E
More examples: Ca
Si, Al
Altogether 20 surface defects (69%) and 2 particulates (7%, Ca, Si, Al) were found.
At 7 emission sites the origin of EFE is not clear (24%)

14. FN-PARAMETERS STATISTICS
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 βFN and SFN
A = 154, B = 6830
[E] = MV/m, [I] = A
Many (βFN, SFN)-pairs within reasonable limits for SFN
Highest reasonable βFN = 103
Many βFN =
Few emitters with too low SFN
assumed φ wrong
Some emitter with much too high SFN
Too low statistics within the FN-plot
Other EFE mechanism (e.g. resonant tunneling)

15. Origin of breakdowns in rf structures
After activation: Field level of IFN = 1 nA is reduced → Eon(1 nA) < Eact
Determination of Eon(1 nA) by local emitter field calibration (U(z)-measurement)
Both samples show similar Eon
Some emitters have very low Eon < 100 MV/m!
Measure for the activation strength: field reduction factor ρ = Eact/Eon
Determination of ρ for 27 emission sites
14 emission sites (52%) show ρ ≥ 1.5
Current is strongly enhanced at these emitters after activation during part of the rf cycle (11 GHz → E > 0.856×Epeak for 15 ps)
Breakdown

16. Examples: Two candidates for breakdowns
Wrong φ?
Activated between 240 – 250 MV/m, Eon = 54 MV/m → ρ = 4.62 ± 0.19
Calculated current at 243 MV/m with β = 17 and S = 1.1×1025 m²! : IFN ~ 1022 A!
Activated between 130 – 140 MV/m, Eon = 80 MV/m → ρ = 1.75 ± 0.12
Calculated current at 243 MV/m with β = 43 and S = 2.4×102 µm² : IFN ~ 3 mA

17. conclusions OUTLOOK N(Eact) increases exponentially
Scaling of EFE loading of rf structures possible (good statistics required)
DIC leads to a significant decrease of N(Eact) on smooth Cu surfaces
Emission sites are mostly surface defects (69%) and particulates (7%, Al, Si, Ca)
Origin of surface defects: Insufficient polishing, inproper handling, …
Origin of particulates: Insufficient final surface cleaning, fast pumping, …
Breakdown of rf cavities probably caused by emitters with field reduction factor ρ > 1.5
OUTLOOK
Reduction of surface defects, e.g. by electropolishing, …
Reduction of particulates by improved DIC parameters (p, t, filter,…)
Improvement of statistics (more samples) for better scaling of EFE loading
Correlated AFM measurements on activated surface defects
Identification / elimination of breakdown relevant emitters (high ρ)

18. Discharge area
17E

19. What is not emitting? Ca Ca
Ca, P Ca