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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 Laboratory School of Electrical Engineering Tel-Aviv University. Background and Objectives. Vacuum Arc Deposition

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slide1
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 EngineeringTel-Aviv University

Thin Films Applied To Superconducting RF

background and objectives
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

Thin Films Applied To Superconducting RF

outline
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

Thin Films Applied To Superconducting RF

slide4
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

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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

Difference between Glow and Arc –cathode electron emission process

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cathode spot characteristics
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
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

Thin Films Applied To Superconducting RF

cathode spot theory
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
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

Plasma Jet Expansion

Cathode Spot

Region

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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

Beilis Model

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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

Beilis Model – cont’d

Thin Films Applied To Superconducting RF

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

Beilis Model –Hydrodynamic Plasma Expansion

Thin Films Applied To Superconducting RF

explosive electron emission mesyats et al
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
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.

Thin Films Applied To Superconducting RF

macroparticles
Macroparticles

Thin Films Applied To Superconducting RF

macroparticles16
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

Thin Films Applied To Superconducting RF

ii vacuum arc engineering
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.)

Thin Films Applied To Superconducting RF

ii vacuum arc engineering18
II. Vacuum Arc Engineering
  • Arc Ignition
  • Cathode Spot Confinement and Motion
  • Heat Removal
  • Macroparticle Control

Thin Films Applied To Superconducting RF

arc ignition
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

Thin Films Applied To Superconducting RF

controlling cathode spot location and motion
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

Thin Films Applied To Superconducting RF

magnetic control of cathode spots
Magnetic Control of Cathode Spots

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passive border
Passive Border

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strelnitski shield
Strelnitski Shield

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pulse control
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

Thin Films Applied To Superconducting RF

heat removal
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:

Thin Films Applied To Superconducting RF

heat removal from cathode
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

Thin Films Applied To Superconducting RF

heat removal from cathode cont d
Heat Removal from Cathode, cont’d
  • Then average surface Temp (far from C.S.) given by

hc– contact heat transfer coefficient

hw– heat transfer coefficient to water

Thin Films Applied To Superconducting RF

substrate temperature control
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

Thin Films Applied To Superconducting RF

macroparticle control
Macroparticle Control
  • 3 Approaches
    • Ignore
      • Get good results (e.g. with tool coatings) despite (or because of?) MPs
    • Minimize MP Production/Transmission
    • Remove MPs

Thin Films Applied To Superconducting RF

minimize mp production transmission
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

Thin Films Applied To Superconducting RF

macroparticle removal
Macroparticle Removal
  • Filtered Vacuum Arc Deposition
    • Use magnetic field to bend plasma beam around an obstacle which blocks MP transmission

Thin Films Applied To Superconducting RF

slide35
Two quarter-torus filtered arcs at Tel Aviv University

Thin Films Applied To Superconducting RF

filtered arc advantages and disadvantages
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

Thin Films Applied To Superconducting RF

other arc modes
Other Arc Modes
  • Hot Anode Vacuum Arc
    • Crucible anode
  • Hot Refractory Anode Vacuum

Thin Films Applied To Superconducting RF

slide40
10 mm

Thin Films Applied To Superconducting RF

iii how can we coat the inside of
III. How can we coat the inside of:

Thin Films Applied To Superconducting RF

approach 1 ignore mps
Approach 1: Ignore MPs

Thin Films Applied To Superconducting RF

approach 1 ignore mps43
Approach 1: Ignore MPs
  • Cavity serves as vacuum chamber and anode
  • Various techniques for magnetically controlling c.s. motion

Thin Films Applied To Superconducting RF

approach 2 miniature filter example welty rect filter
Approach 2: Miniature Filter:Example – Welty Rect. Filter

Thin Films Applied To Superconducting RF

approach 2 miniature filter another example
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!

Thin Films Applied To Superconducting RF

approach iii beilis black body hrava deposition device
Approach III. Beilis “black-body” HRAVA deposition device

Thin Films Applied To Superconducting RF

interior coatings considerations
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

Thin Films Applied To Superconducting RF

summary and conclusions
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

Thin Films Applied To Superconducting RF

summary and conclusions cont d
Summary and Conclusions, cont’d
  • Several approaches exist for efficiently and rapidly coating interior of RF cavities
    • But with technical difficulties

Thin Films Applied To Superconducting RF

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