thin films applied to superconducting rf 1 vacuum arc deposition in interior cavities physical and...

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

thin films applied to superconducting rf 1 vacuum arc deposition in interior cavities physical and...

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