dc breakdown measurements sergio calatroni present team: jan kovermann, chiara pasquino, rocio...
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DC breakdown measurements
Sergio CalatroniPresent team: Jan Kovermann, Chiara Pasquino, Rocio Santiago Kern, Helga
Timko, Mauro Taborelli, Walter Wuensch
Outline• Experimental setup• Typical measurements• Materials and surface preparations• Time delays before breakdown• Gas released during breakdown• Evolution of and Eb• Effect of spark energy• Future
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Experimental set-up : ‘‘ the spark system ’’
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Vpower supply (up to 15 kV)
C (28 nF typical)
vacuum chamber (UHV 10-10 mbar)
anode(rounded tip,
Ø 2 mm)
cathode(plane)
HV switch HV switch
spark
m-displacementgap 10 - 50 mm
(±1 mm)20 mm typically
• Two similar systems are running in parallel
• Types of measurements : 1) Field Emission ( b)
2) Conditioning ( breakdown field Eb)
3) Breakdown Rate ( BDR vs E)
3
Experimental set-up : diagnostics
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V V
HV probe
to scope
current probe
vacuum gauges
V
gas analyzer
optical fibre
photomultiplieror spectrometer
4
Field emission - measurement• An I-V scan is performed at limited current, fitting the data to
the classical Fowler-Nordheim formula, where [jFE] = A/m2, [E] = MV/m and [φ] = eV (usually 4.5 eV).
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E
EjFE
2/332/1
26 1053.6exp)41.10exp(
)(1054.1
0.0065 0.007 0.0075 0.008 0.0085-35
-34
-33
-32
-31
-30
-29ln(I/E^2)
Linear (ln(I/E^2))
1/E
Log(I/E^2)
is extracted from the slope
Conditioning – average breakdown field
Molybdenum Copper
Conditioning phase: 40 sparks
Deconditioning 1-5 sparks or no conditioning
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Eb
Eb
Surface damage (Mo)
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1 5 10
20 40 100
Conditioning curves of pure metals
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Selection of new materials for RF structure fabrication was the original purpose of the experiment
Breakdown field of materials (after conditioning)
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• In addition to other properties, also importance of crystal structure?• reminder : Cu < W < Mo same ranking as in RF tests (30 GHz)
hc
p
hc
p bc
c
bc
c
bc
c
bc
c
bc
c
bc
c
fcc
fcc
fcc : face-centered cubicbcc : body-centered cubichcp : hexagonal closest packing
9
Surface treatments of Cu
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• Surface treatments on Cu only affects the very first breakdowns
rolled sheet / heat treatm. milling Subu electro-polishing
b before 1st spark ~ 15 - 20 ~ 20 ~ 25 - 30 ~ 15 - 20
1st brkd field [MV/m] ~ 200 - 400 ~ 300 - 500 ~ 150 - 200 ~ 300 - 400
• After a few sparks: ~ 170 MV/m, b ~ 70 for every samples
The first sparks destroy rapidly the benefit of a good surface preparation and result in deconditioning. This might be the intrinsinc property of copper surface
In RF, sparks are distributed over a much larger surface, and conditioning is seen. Might be due to extrinsic properties.
• More foreseen in the near future to assess the effect of etching, brazing, etc.
10
Oxidized copper
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Cu2O is a p-type semiconductor, with a higher work function than Cu : 5.37 eV instead
of 4.65 eV
1. Cu oxidized at 125°C for 48h in oven (air): purple surface ↔ Cu2O layer ~15 nm
2. Cu oxidized at 200°C for 72h in oven (air):
BDR = 1 for standard Cu @ 300 MV/m
BDR = 10-3 – 10-4 for oxidized Cu @ 300 MV/m, but last only a few sparks
1 2
11
Breakdown rate experiments
• A target field value is selected and applied repeatedly for 2 seconds• BDR is as usual: #BD / total attempts• Breakdown do often appear in clusters (a simple statistical approach can
account for this)
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Breakdown Rate : DC & RF (30 GHz)
g DC RF
Cu 10 - 15 30
Mo 30 - 35 20
BDR ~ Eg Same trend in DC and in RF,
difficult to compare ‘slopes’
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Time delays before breakdown
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delay• Voltage rising time : ~ 100 ns
• Delay before spark : variable
• Spark duration : ~ 2 ms
14
Time delays with different materials
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Cu Ta Mo SS
R = fraction of delayed breakdowns (excluding conditioning phase, where
imediated breakdowns dominate)
R = 0.07 R = 0.29 R = 0.76 R = 0.83
R increases with average breakdown field
Eb = 170 MV/m Eb = 300 MV/m Eb = 430 MV/m Eb = 900 MV/m
(but why ?!?)
15
Correlation pre-current & delays
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From: Kartsev et al. SovPhys-Doklady15(1970)475
With our simple thermal model (based on Williams & Williams J. Appl. Phys. D 5 (1972) 280) which lead to the establishment of the scaling quantity “Sc” Phys.Rev.ST-AB 12 (2009) 102001
Gas released during a breakdown
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0.8 J / spark0.95 J / spark
• Same gases released, with similar ratios
Outgassing probably dominated by Electron Stimulated Desorption (ESD)
• Slight decrease due to preliminary heat treatment
• Data used for estimates of dynamic vacuum in CLIC strucures
(heat treatment: ex-situ, 815°C, 2h, UHV)
17
H2 outgassing in Breakdown Rate mode (Cu)
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‘quiet’ period
consecutivebreakdowns
Outgassing peaks at breakdowns
Slight outgassing during ‘quiet’ periods ESD with FE e- at the anode
No visible increase in outgassing just before a breakdown
18
Local field: · Eb (Cu)
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• Measurements of b after each sparks (Cu electrodes)
b ↔ next
E b
correlatio
n
b · Eb = const
b ↔ previous E
b
no correlation
b · Eb is the constant parameter
(cf. Alpert et al., J. Vac. Sci. Technol., 1, 35 (1964))19
Evolution of & Eb during conditioning experiments
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(± 32%)
(± 36%)
Eb = 159 MV/m
b = 77
Local field = const = 10.8 GV/m for Cu
(± 16%)
b · Eb = 10.8 GV/m
conditioning ?
good surface state
20
Evolution of during BDR measurements (Cu)
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Spark
• General pattern : clusters of consecutive breakdowns / quiet periods (BDR = 0.11 in this case)
• b slightly increases during a quiet period if E is sufficiently highThe surface is modified by the presence of the field (are « tips » pulled?)
No spark
21
Evolution of during BDR measurements (Cu)
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• Breakdown as soon as b > 48 ( ↔ b · 225 MV/m > 10.8 GV/m)
• Consecutive breakdowns as long as b > bthreshold
length and occurence of breakdown clusters ↔ evolution of b
b·E = 10.8 GV/m
Spark
No spark
22
Effect of spark energy - Cu
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• EBRD increases with lower energy (less deconditioning is possible)
• Local breakdown field remains constant
Effect of spark energy - Mo
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• EBRD seems to increase with higher energy (better conditioning possible)
• Local breakdown field remains constant
• However, we have doubts on representative the measurement is in this case
• The diameter of the damaged area depends on the energy available– Area mostly determined by the conditioning phase– Decreases with decreasing energy; saturates below a given threshold
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Cu Mo
25
Energy scaling of the spot size
The future• Ongoing work:
– Finalise the work on the effect of spark energy on BD field, and understand the beta measurements for Mo
– Trying to understand “worms”( “flowers”?)
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The future• Effect of temperature on evolution and other
properties– To verify the hypothesis and the dynamic of dislocation
• Fast electronics– Defined pulse shape and duration, without and during
breakdowns– Repetition rate up to 1 kHz (107 pulses -> less than 3 hours)
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Fast electronics
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Mike BarnesRudi Henrique Cavaleiro Soares
The future• Effect of surface treatment and in general of the
fabrication process on BD
– To study the influence of etching and its link with machining (preferential etching at dislocations, field enhancement or suppression, smoothening etc.)
– To study the influence of H2 bonding (faceting, etc)– (In parallel, ESD studies on the same samples)
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Depends on capability of
accurately measure th
e gap
without p
rior s
urface contact
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1st oxidation30 sparks2nd oxidation
2 4 6 8 10Energy (keV)
0
50
100
150
cps
O
Cu
Cu
Cu Cu
O element%=1.3-1.7
2 4 6 8 10Energy (keV)
0
50
100
150
cps
OCu
Cu
Cu Cu
O element% = 3.5-3.7
2 4 6 8 10Energy (keV)
0
20
40
60
80
100
cps
OCu
Cu
Cu Cu
O element% = 0.9-2.1
2 4 6 8 10Energy (keV)
0
50
100
150
cps
OCu
Cu
Cu Cu
O element% = 1.6
1st oxidation150 sparks2nd oxidation180 sparks
2 4 6 8 10Energy (keV)
0
50
100
150
cps
OCu
Cu
Cu Cu
O element% = 3.8
2 4 6 8 10Energy (keV)
0
50
100
150
cps
O
Cu
Cu
Cu Cu
O element% = 1.0
2 4 6 8 10Energy (keV)
0
50
100
150
cps
O
Cu
Cu
Cu Cu
O element% = 0.3
31
• A sparked (damaged) surface was reoxidised by heating and was sparked then again
– Was not able to recover the initial high EBRD
– Oxidised, smooth surface high EBRD
– Oxidised, sparked surface no improvement– Connection to the oxidation process?
Reoxidation
Field profile on the cathode surface
• Tip – plane geometry
• dependence with the gap distance
x
E
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b · Eb is the constant parameter
(cf. Alpert et al., J. Vac. Sci. Technol., 1, 35 (1964))
Gap dependence of Eb, b and b · Eb (Cu)
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Surface damage (Cu)
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Gap measurement damage
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Heating of tips by field emission - I
• The tip has a height/radius (field enhancement factor)• For a given value of applied E the Fowler-Nordheim law gives a current density
J(E)=A*(E)2*exp(-B/E)• This current produces a power dissipation by Joule effect in each element dz of the
tip, equal to dP = (J r2)2 (z) dz / r2
• The total dissipated power results in a temperature increase of the tip (the base is assumed fixed at 300 K). The resistivity itself if temperature dependent
• Using the equations we can, for example, find out for a given what is the field that brings the “tip of the tip” up to the melting point, and in what time.
• Used for “Sc” derivation Phys.Rev.ST-AB 12 (2009) 102001
J(E)h=height
r=radius
dz
T = 300K
T = Tmelting
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Heating of tips by field emission - II• If the resistivity is considered temperature-independent, a stable
temperature is always achieved [Chatterton Proc. Roy. Soc. 88 (1966) 231]• If the resistivity (other material parameters play a lesser role) is temperature
dependent, then its increase produces a larger power dissipation, resulting in a further temperature increase and so on [Williams & Williams J. Appl. Phys. D 5 (1972) 280]
• Below a certain current threshold, a stable regime is reached• Above the threshold, a runaway regime is demonstrated• The time dependence of the temperature can be calculated.
High Gradient Workshop 2008 38Sergio Calatroni - CERN
Simulation for Mo cone: diameter 20 nm, beta = 30E=378 MV/mE=374 MV/m
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Time constant to reach the copper melting point (cylinders, =30)
10-4
10-3
10-2
10-1
100
101
102
10-12
10-10
10-8
10-6
10-4
0.01
1
101 102 103 104 105
Parameters to attain the melting point of the tipof a Cu cylinder of given radius and =30
I peak [A]
P peak [W]time constant [sec]
Energy [J]
radius [nm]
The tips which are of interest for us are extremely tiny, <100 nm (i.e. almost invisible even with an electron microscope)
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Power density at the copper melting point (cylinders, =30)
102
103
108
109
1010
1011
1012
1013
1014
101 102 103 104 105
Parameters to attain the melting point of the tipof a Cu cylinder of given radius and =30
E peak [MV/m]Power density [W/m2]Current density [A/m2]
radius [nm]
Power density (power flow) during the pulse is a key issue.See talk by A. Grudiev for RF structures scaling based on Poyinting vector