Transcript Boxman_Padua Lect - LNL-INFN

VACUUM ARC DEPOSITION
IN INTERIOR CAVITIES
Physical and Engineering Principles
and Ideas for Interior Implementations
Raymond L. Boxman
Electrical Discharge and Plasma Laboratory
School 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
V (V)
corona
– Glow Discharge
1000
• V ~ 100’s V, I ~mA’s
• Cathode fall 150-550 V,
depends on gas and
cathode material
glow
100
arc
0.001
1
I(A)
– Arc
• 10’s of volts, A-kA
• Cathode spots
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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 -JB 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 &
Cathode Plasma Jet
Electron relaxation zone.
Ion diffusion
Cathode
SHEATH
Hydrodynamic Plasma Flow
Acceleration Region
e i
e a

Kinetic flow
Knudsen
Layer
Cathode Spot
Region
Plasma Jet
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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
thermaldirected
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
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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
Plasma diverted
in JXB direction
B
B
J
CS moves in
-JXB direction
-
-
CS moves in
direction of
acute angle
FAVORED
C.S.
LOCATION
FAVORED
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Passive Border
WALL
BN BORDER
CATHODE
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Strelnitski Shield
CA THO DE
S HI E LD
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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  at J iV p
Vp  (Ek  ZVb  Ei  Ev  Zw )
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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
S
T  To 
k1 L1  hc  k2 L2  hw
hc – contact heat transfer coefficient
hw – heat transfer coefficient to water
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L1 L2
water interface
contact interface
S
T  To 
k1 L1  hc  k2 L2  hw
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CONTACT
AREA
point
contacts
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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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Macroparticle Removal
• Filtered Vacuum Arc Deposition
– Use magnetic field to bend plasma beam
around an obstacle which blocks MP
transmission
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VENETIAN BLIND
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FILTERED SOURCE DESIGNS
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INTERNAL COIL
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1 cm
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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
+
+
+
Anode
+M -
-
+
-
P flu
x
+
-
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+
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-
Anode
+
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-+
+-
Cathode
-
lux
MP f
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Plasma
+
Plasma
• Hot Refractory Anode Vacuum
+
-
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+
+
-
-
+
-
-
Deposited
sample Water
Cathode
and anode
shields
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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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