thin films applied to superconducting rf 1 vacuum arc deposition in interior cavities physical and...
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Thin Films Applied To Superconducting RF
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VACUUM ARC DEPOSITION IN INTERIOR CAVITIES
Physical and Engineering Principles and Ideas for Interior Implementations
Raymond L. Boxman
Electrical Discharge and Plasma LaboratorySchool of Electrical Engineering
Tel-Aviv University
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Background and Objectives• Vacuum Arc Deposition
– (a.k.a. cathode arc deposition, arc evaporation)– Most popular method for applying hard coatings
in tool industry– …but less well known than other PVD (e.g.
sputtering, e-beam evaporation) and CVD methods
• Objectives of this lecture:– Review:
• Physics of vacuum arc• Engineering issues in vacuum arc deposition
– Suggest implementations with interior cavity
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Outline
• I. Physics of the Vacuum Arc– The Arc Discharge– Cathode Spots and Cathode Spot Plasma Jets
• Observations• Theory
– Macroparticles• II. Vacuum Arc Engineering
– Arc Ignition– Cathode Spot Confinement and Motion– Heat Removal– Macroparticle Control
• III. Suggestions for Coating Interior Cavities
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I. Physics of the Vacuum Arc – The Arc Discharge
• D.C. Discharges– Corona
• High V, Low I• At sharp point
– Glow Discharge• V ~ 100’s V, I ~mA’s• Cathode fall 150-550 V,
depends on gas and cathode material
– Arc• 10’s of volts, A-kA• Cathode spots
I(A)
V (V)
corona
glow
arc
10.001
100
1000
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Difference between Glow and Arc – cathode electron emission process
Glow• ‘individual’ secondary
emission of electrons by:– Ions (depends on ionization
energy, not kinetic energy)– Excited Atoms– Photons
• Not enough!– Multiplication in
avalanche near cathode– Need high cathode drop
(100’s of V’s)– Used in sputtering to
accelerate bombarding ions into ‘target’ cathode
Arc• Collective electron
emission– Current at cathode
concentrated into cathode spots
– Combination of thermionic and field emission of electrons
– Can get sufficient electron current
– Low cathode voltage drop (10’s of V’s)
– High temp. in cathode spot gives high local evaporation rate – used in vacuum arc deposition
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Cathode Spot Characteristics
• Diam: m’s• Lifetime: ns’s to
s’s– Extinguish, reignite
at adjacent location– Apparent ‘random
walk’ motion– In B field,
“retrograde motion” in -JB direction
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Cathode Spot Plasma Jets
• ~Fully Ionized– Multiple ionizations common
• Zav(Ti) ~2
• Ion directed kinetic energy 20-150 eV– Flow velocity ~104 m/s
• ~cos distribution• Ti, Te ~few eV• Supersonic ions, thermal electrons• Ii -0.1 Iarc, Ie 1.1 Iarc
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Cathode Spot Theory
• Two Approaches:– Quasi-continuous (~steady state)– Explosive Emission
• Quasi-continuous approach:– Must account simultaneously for:
• Cathode heating (for e- and atom emission)• Electron emission• Atom emission• High ion energy / plasma velocity
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Beilis Model: Cathode Spot & Beilis Model: Cathode Spot & Cathode Plasma JetCathode Plasma Jet
Cathode
Cathode SpotRegion
Hydrodynamic Plasma Flow
SHEATH
Electron relaxation zone.Ion diffusion
Kinetic flowKnudsen Layer
Plasma Jet Expansion
Acceleration Region e i e a
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Beilis Model• TF emission of
electrons• Evaporation of
atoms• Acceleration of
electrons into vapor– Collisionless sheath– Collisionless
Knudsen layer– Electrons loose
energy to vapor in relaxation zone
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Beilis Model – cont’d• Back-flow of
electron and ions to cathode– Heats cathode spot
• Joule heating under cathode surface
• Joule heating of plasma
• Hydrodynamic plasma expansion
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Beilis Model – Hydrodynamic Plasma Expansion
• Like in jet engine – conversion of thermaldirected kinetic energy
• But plasma heated all along length– Continuous heating,
conversion into kinetic energy, so
• Ti~3ev, • Ei~20-150eV
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Explosive Electron Emission (Mesyats et al.)
• Cathode spot is a sequence of explosion of protuberances
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EEE (Mesyats et al.) – cont’d
• Each explosion creates further protuberances, which can then explode
• Idea supported by high resolution laser shadowgraphs, showing short life time and small dimensions, spike noise in ion current, etc.
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Macroparticles
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Macroparticles• Spray of liquid metal
droplets from the cathode spot
• small fraction of cathode erosion for refractory metals
• large fraction of cathode erosion for low melting point cathode materials
• exponentially decreasing size distribution function
• most mass in the 10-20 m diam range
• preferentially emitted close to cathode plane
• Downward pressure from plasma jet on liquid surface
CATHODE
CATHODESPOT
PLASMAJET
MP SPRAY
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II. Vacuum Arc Engineering
• Coating forms on any substrate intercepting part of plasma jet
• In vacuum, coating composition cathode composition
• In reactive gas background, can form compounds (nitrides, oxides, carbides, etc.)
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II. Vacuum Arc Engineering
• Arc Ignition
• Cathode Spot Confinement and Motion
• Heat Removal
• Macroparticle Control
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Arc Ignition• Problem: extremely high voltage needed to
break-down vacuum gap (~100 kV/cm)• Drawn-arc (most common)
– Trigger electrode, mechanically operated– Connected to +voltage (e.g. anode via current
limiting resistor)– Momentary contact with cathode– Arc ignited when contact broken
• Current transfers to main anode
• Breakdown to trigger electrode– Vacuum gap– Sliding spark
• Laser ignition
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Controlling Cathode Spot Location and Motion
• Objectives:– Locate CS’s on ‘front’ surface of cathode
• Maximize plasma transmission to substrates• Prevent damage to cathode structure
– Methods:• Magnetic Field (retrograde and “acute angle” motion• Passive border• Strellnitski shield• Pulsed arc
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Magnetic Control of Cathode Spots
B
J
Plasm a divertedin JXB direction
CS m oves in-JXB direction
- -
B
CS m oves indirection ofacute angle
FAVO REDC.S.LO CATIO N
FAVO REDPATH FO R C.S.'s
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Passive Border
W ALLBN B O R D ER
C ATH O D E
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Strelnitski Shield
CATHODE
SHIELD
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Pulse Control
• Basic Idea: arc duration shorter than CS travel time to edge– Short Pulse– Laser Ignition– Long Pulse - Long Cathode– Active detection of CS location –
• quench arc when CS reaches edge
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Heat Removal• Total power P = VarcIarc
– Varc ~20-40 V– Iarc ~ 50-1000 A– P > 1 kW
• Distribution– ~1/3 in cathode– ~2/3 in anode– Substrate: S a J Vt i p
)( wvibkp ZEEZVEV
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Heat Removal from Cathode
• Cool cathode important to– minimize MP generation– Prevent cathode damage
• In best case, C.S.’s rapidly moved around to give on average a uniform heat flux on cathode surface S=P/A
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Heat Removal from Cathode, cont’d
• Then average surface Temp (far from C.S.) given by
T TS
k L h k L hoc w
1 1 2 2
hc – contact heat transfer coefficient
hw – heat transfer coefficient to water
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L1 L2contact interface
water interface CONTACTAREA
pointcontactsT T
S
k L h k L hoc w
1 1 2 2
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Substrate Temperature Control
• Ts critical in determining coating properties
• Measure with IR radiation detector• Ts determined by balance between
heating and cooling processes• Often use heat flux from process to
control Ts – Vary bias or arc current
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Macroparticle Control
• 3 Approaches– Ignore
• Get good results (e.g. with tool coatings) despite (or because of?) MPs
– Minimize MP Production/Transmission– Remove MPs
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Minimize MP Production/Transmission
• Choose refractory cathode material– “Poison” (i.e. nitride) cathode surface
• Operate at ‘higher’ N2 background pressure
• Low cathode temperature– direct cooling– lower current (lower deposition rate)
• Place substrates where plasma flux max, MP flux min
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CATHODE
CATHODESPOT
PLASMAJET
MP SPRAY
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Macroparticle Removal
• Filtered Vacuum Arc Deposition– Use magnetic field to bend plasma beam
around an obstacle which blocks MP transmission
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STRAIGHTDUCT
BENTBEAM KNEE 1/4-TORUS DOME
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C
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INTERNAL COIL
FILTERED SOURCE DESIGNS
VENETIAN BLIND
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Two quarter-torus filtered arcs at Tel Aviv University
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Filtered Arc – Advantages and Disadvantages• Advantages
– High quality, very smooth coatings– ‘almost’ MP free– Can achieve higher deposition rate than other
‘high quality’ techniques
• Disadvantages– Usually poor plasma transmission
• Material utilization efficiency low
– Much slower than unfiltered arc deposition– Bulky equipment
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Other Arc Modes
• Hot Anode Vacuum Arc– Crucible anode
• Hot Refractory Anode Vacuum
MP flux
Plasm
a+
- + -
+ -
+
- + -
+
-
+
-
-+
+
-
Anode
Cathode
Anode
Water
MP flux
Pla
sma
Cathodeand anode
shields
Depositedsample
+
-+-
+-
+
-+-
+
-
+
-
-+
+
-+-
+-
+
-
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10 m
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III. How can we coat the inside of:
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Approach 1: Ignore MPs
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Approach 1: Ignore MPs
•Cavity serves as vacuum chamber and anode
•Various techniques for magnetically controlling c.s. motion
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Approach 2: Miniature Filter:Example – Welty Rect. Filter
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Approach 2: Miniature Filter:Another Example
• Progress in Use of Ultra-High Vacuum Cathodic Arcs for Deposition of Thin Film Superconducting Layers
• J.Langner, M.J.Sadowski, P.Strzyzewski, R.Mirowski, J.Witkowski, S.Tazzari, L.Catani, A.Cianchi, J.Lorkiewicz, R.Russo, T.Paryjczak, J.Rogowski, J.Sekutowicz
• Presentation 28 Sept at XXXIII-ISDEIV in Matsue, Japan
• Showed use of a cylindrical “Venetian Blind” filter to deposit Nb inside cavity!
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Approach III. Beilis “black-body” HRAVA deposition device
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Interior Coatings - Considerations
• Use cavity as vacuum chamber– Need complicated end seal to allow for electrical
connections (main arc and trigger), cooling water, in some cases motion
– Cooling can be applied directly to outside of tube
• Fitting everything into cavity – difficult!• Integrity, lifetime?• Triggering – not shown
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Summary and Conclusions
• VAD uses inherent properties of cathode spot plasma jets to rapidly deposit dense, high quality coatings
• Successful implementation requires “plasma engineering” to:– Confine cathode spots on desired surface– Remove process heat– Control macroparticle contamination
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Summary and Conclusions, cont’d
• Several approaches exist for efficiently and rapidly coating interior of RF cavities– But with technical difficulties
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