dc breakdown measurements
DESCRIPTION
DC breakdown measurements . Sergio Calatroni Present 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 - PowerPoint PPT PresentationTRANSCRIPT
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DC breakdown measurements
Sergio CalatroniPresent team: Jan Kovermann, Chiara Pasquino, Rocio Santiago Kern, Helga
Timko, Mauro Taborelli, Walter Wuensch
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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 ( )
2) Conditioning ( breakdown field Eb)
3) Breakdown Rate ( BDR vs E)
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Experimental set-up : diagnostics
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V VHV probe
to scope
current probe
vacuum gauges
V
gas analyzer
optical fibre
photomultiplieror spectrometer
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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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EEjFE
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
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Conditioning – average breakdown field
Molybdenum Copper
Conditioning phase: 40 sparks
Deconditioning 1-5 sparks or no conditioning
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Eb
Eb
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Surface damage (Mo)
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1 5 10
20 40 100
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Conditioning curves of pure metals
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Selection of new materials for RF structure fabrication was the original purpose of the experiment
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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)
hcp
hcp bc
c
bcc
bcc
bcc
bcc
bcc
fcc
fcc
fcc : face-centered cubicbcc : body-centered cubichcp : hexagonal closest packing
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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
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, ~ 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.
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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
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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
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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 ?!?)
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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
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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)
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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
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Local field: · Eb (Cu)
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• Measurements of after each sparks (Cu electrodes)
↔ next E
b
correlatio
n
· Eb = const
↔ previous Eb
no correlation
· Eb is the constant parameter
(cf. Alpert et al., J. Vac. Sci. Technol., 1, 35 (1964))19
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Evolution of & Eb during conditioning experiments
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(± 32%)
(± 36%)
Eb = 159 MV/m
= 77
Local field = const = 10.8 GV/m for Cu
(± 16%)
· Eb = 10.8 GV/m
conditioning ?
good surface state
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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)
• 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
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Evolution of during BDR measurements (Cu)
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• Breakdown as soon as > 48 ( ↔ · 225 MV/m > 10.8 GV/m)• Consecutive breakdowns as long as > threshold
length and occurence of breakdown clusters ↔ evolution of
·E = 10.8 GV/m
Spark
No spark
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Effect of spark energy - Cu
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• EBRD increases with lower energy (less deconditioning is possible)
• Local breakdown field remains constant
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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
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• 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
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Energy scaling of the spot size
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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
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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 the gap
without prior surface contact
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1st oxidation30 sparks2nd oxidation
2 4 6 8 10Energy (keV)
0
50
100
150
cps
OCu
Cu
Cu Cu
O element%=1.3-1.7
2 4 6 8 10Energy (keV)
0
50
100
150cps
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
OCu
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
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• 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
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Field profile on the cathode surface
• Tip – plane geometry
• dependence with the gap distance
x
E
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· Eb is the constant parameter
(cf. Alpert et al., J. Vac. Sci. Technol., 1, 35 (1964))
Gap dependence of Eb, and · 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.
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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