VACUUM ARC DEPOSITION Principles of Vacuum Arc Deposition (VAD) Cathode Spots Reactive VAD Magnetic Field in VAD Filtered ...
VACUUM ARC DEPOSITION (VAD) <ul><li>VACUUM ARC DEPOSITION (VAD)   is a  PVD  process in which the  cathode  (or the  somet...
CATHODE-SPOT VACUUM ARC <ul><li>CATHODE-SPOT VACUUM ARC   is a  high current ,  low voltage  electrical discharge, sustain...
CATHODE SPOT VACUUM ARC Example:  Hot Refractory Anode Vacuum Arc (HRAVA), Movie 1 <ul><li>EXPERIMENTAL SET-UP </li></ul><...
CATHODE SPOT VACUUM ARC DISCHARGE <ul><li>Initiation of a high current, low voltage discharge between two  electrodes of c...
CATHODE SPOTS <ul><li>Cathode spots (CS) appear on the cathode surface at discharge </li></ul><ul><li>CS’s are a  highly l...
EROSION OF MATERIAL FROM CATHODE SPOTS <ul><li>CS momentarily location – local material melting and  erosion of the cathod...
CATHODE EROSION TRACE <ul><li>Mo cathode </li></ul><ul><li>Chain of erosion craters on the cathode surface after cathode s...
CATHODE SPOT EROSION SPECIES AND VACUUM ARC PLASMA <ul><li>Charged particles (electrons, ions) – highly ionized plasma jet...
CATHODE SPOT  VACUUM ARC PLASMA <ul><li>All conducting materials may produce vacuum arc plasma from the cathode spots </li...
VACUUM ARC PLASMA   FROM CATHODE SPOTS TO THE SUBSTRATE  VACUUM ARC DEPOSITION (VAD) Tracks of Cathode Spot Motion Multipl...
VACUUM ARC PLASMA CHARACTERISTICS <ul><li>Metal plasma flow  of cathode material – positive metal ions and electrons </li>...
MULTIPLE IONIZATION: EXAMPLE Ti Plasma in Vacuum, Arc Current 100 A <ul><li>Ti ions in plasma: Ti + , Ti 2+ , Ti 3+ </li><...
VACUUM ARC PLASMA CHARACTERISTICS   <ul><li>Quasi neutral plasma :  N e =Z i N i   </li></ul><ul><li>N e  and N i  – elect...
COLD CATHODE – HOT CATHODE <ul><li>Momentarily cathode spot appearance on some location on the cathode surface –  local te...
VACUUM ARC PLASMA JET <ul><li>VACUUM ARC PLASMA JET  – metal plasma flow produced by the cathode spots –  positively ioniz...
MACROPATICLES  IN PLASMA JET <ul><li>MACROPARTICLES , or micro-droplets , are molten droplets of cathode material, produce...
MACROPARTICLES  IN VAD COATINGS
CURRENT-VOLTAGE CHARACTERISTICS  OF A DISCHARGE TUBE
COMPARISON OF PLASMAS IN VACUUM ARC DEPOSITION AND SPUTTERING SYSTEMS: High voltage (few hundreds V - ~1 kV), low current ...
COMPARISON OF PLASMAS (CONTINUE) 10 14 -10 18 10 16 -10 20 Plasma density, m -3 (3-6)  10 3  (Ar ions) (1-2)  10 4 Ion v...
VACUUM ARC DEPOSITON HISTORY - I <ul><li>End XIXth century – A.W. Wright (mirror coatings),  </li></ul><ul><li>T. Edison –...
VACUUM ARC DEPOSITON HISTORY - II <ul><li>1979-1980 – patent and technology was acquired by Multi-Arc Vacuum Systems Inc. ...
VACUUM ARC DEPOSITON HISTORY - III <ul><li>Middle 1980s – Development of multi-layer and multi-component wear resistant co...
VACUMM ARC DEPOSITION Process Definitions, Terms <ul><li>Currently, the following definitions of  conventional  VAD may be...
TYPICAL VACUUM ARC DEPOSITION (VAD) SYSTEM   <ul><li>Water-cooled  cylindrical  cathode of conducting material </li></ul><...
ARC INITIATION <ul><li>Arc ignition (triggering) –  contact mode  or  breakdown mode </li></ul><ul><li>Contact mode - mome...
ARC OPERATION <ul><li>Arc ignited, cathode spots operate on the cathode surface </li></ul><ul><li>Part of electrons  follo...
CATHODE SPOTS <ul><li>Spots operating on the cathode surface produce flux of charged particles </li></ul><ul><li>Spots rap...
ARC CURRENT (I arc ) AND ION CURRENT (I ion ) PRODUCED BY VACUUM ARC PLASMA SOURCE <ul><li>Larger I arc -> and larger numb...
ARC POWER AND HEAT FLUX <ul><li>Arc power: P arc =I arc    V arc </li></ul><ul><li>V arc =15-50 V – arc voltage </li></ul...
COMMERCIAL VACUUM ARC DEPOSITION APPARATUS “Bulat-3”  - NEW EPOCH IN CUTTING TOOL INDUSTRY (former USSR, 1981) Anode – wat...
SCHEME OF COMMERCIAL VAD SYSTEM (“Bulat”- Type)
SCHEME OF COMMERCIAL TRIPLE CATHODE VAD SYSTEM
TRIPLE CATHODE VAD SYSTEM AT TAU
COATINGS TYPES THAT MAY BE FABRICATED USING VAD TECHNIQUES <ul><li>Coatings deposition of most conducting materials: pure ...
COATINGS TYPES THAT MAY BE FABRICATED USING VAD TECHNIQUES (Continue) <ul><li>Multi-layer coatings - at consequent deposit...
REACTIVE VACUUM ARC DEPOSITION <ul><li>Reactive VAD – process of  compound  coating deposition as result of reaction of ca...
REACTIVE VAD - HARD WEAR RESISTANT COATING DEPOSITION   <ul><li>Since beginning of 1980s Reactive VAD is widely used for w...
REACTIVE VAD OF TiN COATING Model by M.Sakaki & T.Sakakibara,  IEEE. Trans. Plasma Sci.,  vol. 22 (1994), p. 1049-1054 <ul...
ION PLATING – SUBSTRATE BOMBARDMENT BY POSITIVE PLAMA IONS ION PLATING IN VAD Role of  negative bias voltage  on  surface ...
NEGATIVE BIAS VOLTAGE APPLIED  TO THE SUBSTRATE IN VAD PROCESS Negative bias voltage to the substrate relative the grounde...
COATING DEPOSITION RATE vs BIAS VOLTAGE TO THE SUBSTRATE <ul><li>Low negative bias voltage to the substrate  |-V bias | – ...
BIAS VOLTAGE APPLICATION: ACTION OF HIGH |V bias | <ul><li>HIGH NEGATIVE V bias  –  SPUTTERING: </li></ul><ul><li>Effectiv...
BIAS VOLTAGE APPLICATION: ACTION OF LOW |V bias | <ul><li>LOW |V bias | -  COATING DEPOSITION </li></ul><ul><li>Nucleation...
INFLUENCE OF BIAS VOLTAGE ON COATING STRUCTURE AND PROPERTIES <ul><li>Bias voltage determines: </li></ul><ul><li>Structura...
BIAS VOLTAGE: CONVENTIONAL COATING DEPOSITION PROCESS <ul><li>CONVENTIONAL DEPOSITION PROCESS, AS A RULE, CONSISTS OF TWO ...
POSSIBLE NEGATIVE EFFECTS OF BIAS APPLICATION TO THE SUBSTRATE <ul><li>Appearance of spots on the  negatively biased subst...
CALCULATION OF HEAT FLUX TO THE SUBSTRATE <ul><li>Heat flux to the substrate, Q </li></ul><ul><li>Q=J i  (|V bias |+E i )...
DEBYE LENGTH <ul><li>Penetration depth of electric field produced by  bias voltage  applied to the substrate –  </li></ul>...
ION FLUX TO THE SUBSTRATE IN VAD  - EFFECTIVENESS OF DEPOSITION PROCESS <ul><li>Measurement of saturated ion current colle...
MEASUREMENT OF ION FLUX TO THE SUBSTRATE (ION CURRENT) Negatively biased Langmuir probe –  saturation ion current measurem...
VOLTAGE-CURRENT CHARACTERISTICS  OF LANGMUIR PROBE Evaluation of deposition rate –  saturated ion current is important
SATURATION ION CURRENT AND DEPOSITION RATE <ul><li>Evaluation of coating deposition rate v d </li></ul>J i  – ion current ...
ION CURRENT AND DEPOSITION RATE <ul><li>For VAD systems v d  1…60 nm/s </li></ul><ul><li>Ion current and deposition rate:...
MAGNETIC FIELD IN VACUUM ARC DEPOSITION <ul><li>Cathode spot motion in the magnetic field </li></ul><ul><li>Magnetic guidi...
MAGNETIC FIELD IN VACUUM ARC PLASMA DEPOSITION SYSTEMS EDPL, TAU – System 3
MAIN FUNCTIONS OF MAGNETIC FIELD IN VAD SYSTEMS <ul><li>MAGNETIC FIELD IN THE VICINITY OF THE CATHODE  – to confine (stabi...
CATHODE SPOT MOTION ON THE FACE SURFACE OF CYLINDRICAL CATHODE. NO MAGNETC FIELD <ul><li>Generally,  random  spot motion o...
MAGNETIC FIELD EFFECT ON THE CATHODE SPOT MOTION <ul><li>TWO EFFECTS OF MAGNETIC FIELD  </li></ul><ul><li>ON THE CATHODE S...
1. RETROGRADE  (-E x B) CATHODE SPOT MOTION IN MAGNETIC FIELD <ul><li>Round cathode, annular anode, axial magnetic field <...
2. “ACUTE ANGLE” RULE. CYLINDRICAL CATHODE - TWO CASES OF MAGNETIC FIELD CONFIGURATION <ul><li>Left  – Magnetic field, gui...
EFFECT OF MAGNETIC FIELD ON CATHODE SPOT MOTION <ul><li>“ Acute angle” rule – the spots tend to move towards the acute ang...
“ ARCHED FIELD” - CIRCUMFERENTIAL SPOT MOTION ON DEFINED TRAJECTORY <ul><li>“ Arched” field on the cathode surface  is pro...
SPOT MOTION ON ROUND CATHODE SURFACE (Ti arc, System 2) No magnetic field – random spot motion Acute angle directed outwar...
“ ARCHED FIELD” CYLINDRICAL CATHODE <ul><li>“ Arched magnetic field is produced by magnetic coil positioned in water-coole...
“ ARCHED” FIELD -  RECTANGULAR CATHODE DESIGH Magnetic field  configuration: QuickField TM  plot Cathode spot  Motion trac...
EFFECT OF “ACUTE ANGLE” IN CONE CATHODE DESIGN – SPOT CONFINEMENT IN CATHODE CENTER Cathode Anode Focusing  Coil Guiding C...
MAGNETIC PLASMA GUIDING TO THE SUBSTRATE   IN THE STRAIGHT CYLINDRICAL DUCT   Axial magnetic field produced by coils 2-4 p...
PLOT OF MAGNETIC FIELD LINES PRODUCED BY AXIAL COILS (Axial Cross-Section) <ul><li>QuickField   plot – axisymmetric cross...
PRINCIPLES OF PLASMA GUIDING IN THE MAGNETIC FIELD PLASMA TRANSPORT IN THE STRAIGHT AND CURVILINEAR DUCT
MOTION OF A CHARGED PARTICLE IN MAGNETIC FIELD <ul><li>Motion of an individual charged particle in magnetic field – two ve...
LARMOR RADIUS OF PARTICLE GYRATION <ul><li>Larmor radius, r L : </li></ul><ul><li>r L =v  /  L =v  m/qB </li></ul><ul><...
GUIDING OF A PLASMA JET IN AN AXIAL MAGNETIC FIELD <ul><li>TWO PRINCIPLES OF PLASMA GUIDING: </li></ul><ul><li>Guiding of ...
GUIDING OF A PLASMA JET IN A RELATIVELY WEAK MAGNETIC FIELD <ul><li>In a relatively weak magnetic field (10-30 mT) – elect...
PARTICLE MAGNETIZATION  EXAMPLE <ul><ul><li>Given: axial field B=12 mT. Duct radius – R=8 cm. </li></ul></ul><ul><li>Quest...
ELECTRON CUT-OFF FROM THE ANODE IN MAGNETIC FIELD – UNSTABLE ARCING <ul><li>Annular anode </li></ul><ul><li>Axial magnetic...
SPATIAL DISTRIBUTION OF THE PLASMA BEAM IN MAGNETIC FIELD <ul><li>Gaussian fitting: Ion current density  J=J o exp(-r 2 / ...
FILTERED VACUUM ARC DEPOSITION (FVAD) <ul><li>Along with charged particles, the plasma flux produced by the cathode spots ...
VAD SYSTEM WITH A QUARTER -TORUS MAGNETIC FILTER Plasma flux is guided by toroidal magnetic field to the substrate positio...
SCHEME OF FILTERED VAD (FVAD) SYSTEM (System 2 at TAU)
MAGNETIC FIELD CONFIGURATION IN THE TOROIDAL DUCT <ul><li>No direct line-of sight from the cathode to the substrate via to...
MACROPARTICLE FILTERING: VAD SYSTEM WITH A QUARTER TORUS FILTER <ul><li>Magnetic field generally  parallel to the duct wal...
COATINGS DEPOSITED WITH UNFILTERED AND FILTERED VAD Macroparticles in Coating Unfiltered VAD Macroparticle-Free Coating Fi...
“ S-FILTER”   (A. Anders, Berkeley, USA) <ul><li>Better macroparticle removal on expense of lower deposition process effic...
“ OPEN” S-FILTER DESIGN  (A. Anders, Berkeley, USA) TOROIDAL FILTER WITH WALL – part of small macroparticles may  reflect ...
ADVANTAGES AND DISANVANTAGES OF MACROPARTICLE FILTERING <ul><li>ADVANTAGES   </li></ul><ul><li>Macroparticle removal.  </l...
DUAL FILTERED VACUUM ARC PLASMA SOURCE <ul><li>Dual FVAD source – increased deposition effectiveness </li></ul><ul><li>Pos...
LARGE-AREA RECTANGULAR FVAD SYSTEM  (System 4, EDPL) <ul><li>Conceptual application of round design to rectangular system ...
SCHEME OF RECTANGULAR FVAD SYSTEM AND  CATHODE APPLICATION: Transparent conducting SnO 2  coating deposition on flat subst...
HOT REFRACTORY ANODE VACUUM ARC (HRAVA) <ul><li>HRAVA-effective method of  macroparticle-free  coating fabrication </li></...
CATHODE SPOT MOTION ON A LONG CYLINDRICAL CATHODE <ul><li>“ Heliclal Arc” </li></ul><ul><li>Cathode spot motion on a long ...
COMPACT VAD SOURCE WITH CONE CATHODE, TAU, 2004-2008
VACUUM ARC PLASMA EVAPORATION (VAPE)  EXAMPLE: NOT CATHODIC VAD! <ul><li>Paper: T. Minami, S. Ida and T. Miyata, “High rat...
LITERATURE <ul><li>R.L. Boxman, D. Sanders and P. Martin, Handbook of Vacuum Arc Science and Technology, Noyes Publication...
Multi-Source VAD Systems Hybrid Deposition Systems
MULTI-SOURCE VAD SYSTEMS <ul><li>Industrial VAD systems are often equipped with few cathode spot plasma sources. This allo...
INDUSTRIAL SYSTEM WITH DUAL FILTERED VAD SOURCE   V. Gorokhovsky, SCT 2004
COMBINED (HYBRID) DEPOSITION SYSTEMS <ul><li>Different deposition processes have their advantages and disadvantages </li><...
HYBRID DEPOSITION SYSTEM BY V. GOROHOVSKY V. Gorokhovsky et. Al., SCT 1993 Substrate Filtered VAD  Source E-Beam Source
LARGE HYBRID SYSTEM WITH VAD AND MAGNETRON SPUTTERING SOURCES CemeCon AG (Germany) By the courtesy of Dr. Christoph Shiffe...
HYBRID VAD-CVD SYSTEM FOR SUPERHARD COATINGS  nc -MeN/ a -Si 3 N 4 <ul><li>NOVEL  SUPERHARD NANOCOMPOSITE  COATINGS   </li...
APPENDIX 1 Pre-Deposition Substrate Ion Cleaning and Ion Plating in Deposition Processes * <ul><li>Pre-Deposition Substrat...
ION BOMBARDMENT IN PVD PROCESSES <ul><li>It was shown (Lectures 5, 6), that substrate surface and growing coating bombardm...
SPUTTER ION CLEANING IN GLOW DISCHARGE PLASMA <ul><li>Sputter ion cleaning of the substrate surface in glow discharge plas...
CATHODIC SPUTTERING: DC TRIODE SYSTEM Negative bias voltage  to the substrate –  Sputter ion cleaning prior to deposition ...
SPUTTER ION CLEANING IN GLOW DISCHARGE PLASMA <ul><li>Diode cathodic sputtering system: </li></ul><ul><ul><li>Short-term c...
ION BOMBARDMENT OF COATING DURING DEPOSITION <ul><li>Positive role of ion bombardment of growing film –  ION PLATING  - wa...
ION BOMBARDMENT IN VACUUM ARC DEPOSION <ul><li>In VAD processes highly ionized  metal plasma  flow is produced at erosion ...
ION BOMBARDMENT IN VACUUM ARC DEPOSION <ul><li>In VAD processes – effective sputter ion cleaning  in vacuum , in contrast ...
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Vacuum arc deposition (Yan Valsky) - Lecture Dr.V.Zhitomirsky (Coating cource/EDPL/TAU)

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Vacuum arc deposition (Yan Valsky) - Lecture Dr.V.Zhitomirsky (Coating cource/EDPL/TAU)

  1. 1. VACUUM ARC DEPOSITION Principles of Vacuum Arc Deposition (VAD) Cathode Spots Reactive VAD Magnetic Field in VAD Filtered Vacuum Arc Deposition Hybrid VAD Systems Substrate Ion Bombardment – Ion Plating Ion Current Measurements Coating Deposition Rate Appendix 1- Ion Plating Lecture 6 2009
  2. 2. VACUUM ARC DEPOSITION (VAD) <ul><li>VACUUM ARC DEPOSITION (VAD) is a PVD process in which the cathode (or the sometimes [rarely ] anode ) vapor plasma produced from high current discharge in vacuum , or in a low-pressure inert gas , or in a reactive gas environment, is transported to a substrate to deposit coating on its surface </li></ul><ul><li>CONVENTIONAL VAD techniques, also named CATHODIC VACUUM ARC DEPOSITION or CATHODE SPOT VACUUM ARC DEPOSITION – vapor plasma of CATHODE material is deposited on the substrate to form the coating on its surface </li></ul>
  3. 3. CATHODE-SPOT VACUUM ARC <ul><li>CATHODE-SPOT VACUUM ARC is a high current , low voltage electrical discharge, sustained between two electrodes – cathode and anode, in which a conducting medium is a highly ionized plasma , generated from luminous regions moving throughout the cathode surface, known as CATHODE SPOTS </li></ul>
  4. 4. CATHODE SPOT VACUUM ARC Example: Hot Refractory Anode Vacuum Arc (HRAVA), Movie 1 <ul><li>EXPERIMENTAL SET-UP </li></ul><ul><li>Vacuum ~10 -5 Torr </li></ul><ul><li>Cathode, Anode 3 cm diam, Electrode separation ~1 cm </li></ul><ul><li>Cathode – Al or Cu </li></ul><ul><li>Refractory Anode – Mo or C </li></ul><ul><li>DC arc current – 200 A </li></ul><ul><li>Arc voltage – 20-22 V </li></ul><ul><li>From experiment by I. Beilis, A.Shashurin, A. Nemirovsky, 2004 (EDPL TAU) </li></ul>
  5. 5. CATHODE SPOT VACUUM ARC DISCHARGE <ul><li>Initiation of a high current, low voltage discharge between two electrodes of conducting materials (cathode and anode) in vacuum – VACUUM ARC DISCHARGE </li></ul><ul><li>Arc discharge ignition – momentarily contact between electrodes, or breakdown </li></ul><ul><li>At arc ignition - cathode spots appear and move on the surface of the cathode </li></ul><ul><li>Vacuum arc discharge may be sustained between electrodes in DC or pulsed mode </li></ul>
  6. 6. CATHODE SPOTS <ul><li>Cathode spots (CS) appear on the cathode surface at discharge </li></ul><ul><li>CS’s are a highly luminous , high current density (10 4 -10 6 A/cm 2 ), high temperature ( 3000-4000 K ) regions rapidly moving on the cathode surface </li></ul><ul><li>CS diameter - 1-100  m </li></ul><ul><li>CS motion velocity – from several m/s and up to ~150 m/s </li></ul><ul><li>Momentarily CS appearance on some location of the cathode surface – local material melting and evaporation - erosion of cathode material </li></ul>
  7. 7. EROSION OF MATERIAL FROM CATHODE SPOTS <ul><li>CS momentarily location – local material melting and erosion of the cathode material </li></ul><ul><li>Erosion products </li></ul><ul><li>Positive ions of cathode material </li></ul><ul><li>Electrons </li></ul><ul><li>Neutrals </li></ul><ul><li>Highly ionized particle flow – VACUUM ARC PLASMA </li></ul>
  8. 8. CATHODE EROSION TRACE <ul><li>Mo cathode </li></ul><ul><li>Chain of erosion craters on the cathode surface after cathode spot operation </li></ul>R. Boxman et al., Ref. [1]
  9. 9. CATHODE SPOT EROSION SPECIES AND VACUUM ARC PLASMA <ul><li>Charged particles (electrons, ions) – highly ionized plasma jet </li></ul><ul><li>Flux produced by the cathode spots, also contains some amount of macro-particles (molten micro-droplets) </li></ul><ul><li>Part of electron flow follows to the anode to sustain discharge </li></ul>Boxman, Goldsmith, 1987
  10. 10. CATHODE SPOT VACUUM ARC PLASMA <ul><li>All conducting materials may produce vacuum arc plasma from the cathode spots </li></ul><ul><li>Vacuum arc cathode – every conducting material: pure metal, alloy, carbon , etc. </li></ul><ul><li>In most cases of conventional VAD, plasma produced from the cathode spots – METAL PLASMA FLOW </li></ul><ul><li>Metal plasma flow of cathode material – positively charged metal particles – positive ions , and electrons </li></ul><ul><li>( in contrast to plasmas in PACVD/PECVD, ARE and sputtering processes where gaseous plasma, mainly glow discharge plasma is used ) </li></ul>
  11. 11. VACUUM ARC PLASMA FROM CATHODE SPOTS TO THE SUBSTRATE VACUUM ARC DEPOSITION (VAD) Tracks of Cathode Spot Motion Multiple spots Ti, I arc ~100 A Single spot Cu, Iarc~30-50 A Plasma produced by a cathode spot Scheme of Cathodic Vacuum Arc Plasma Deposition
  12. 12. VACUUM ARC PLASMA CHARACTERISTICS <ul><li>Metal plasma flow of cathode material – positive metal ions and electrons </li></ul><ul><li>Quasi-neutral medium – summary charge equal zero </li></ul><ul><li>Highly conducting medium </li></ul><ul><li>Highly ionized plasma jet – up to 99% ionization for refractory materials </li></ul><ul><li>Supersonic plasma jet, jet velocity (directed) - v  10 4 m/s </li></ul><ul><li>Radial expansion of plasma jet </li></ul><ul><li>Ion kinetic energy – 50-150 eV </li></ul><ul><li>Multiple ionization of metallic ions: Me + , Me 2+ , Me 3+ , etc. </li></ul>
  13. 13. MULTIPLE IONIZATION: EXAMPLE Ti Plasma in Vacuum, Arc Current 100 A <ul><li>Ti ions in plasma: Ti + , Ti 2+ , Ti 3+ </li></ul><ul><li>Average charge number, Z , for Ti plasma Z=1.79 (I arc =100A) </li></ul><ul><li>For different metals Z in the range of 1-~3.2 </li></ul>103 34 6 +3 79 39 67 +2 65 65 27 +1 Total energy, E i , eV Average energy, E i /Z i , eV Ion fraction, % Charge state Z i
  14. 14. VACUUM ARC PLASMA CHARACTERISTICS <ul><li>Quasi neutral plasma : N e =Z i N i </li></ul><ul><li>N e and N i – electron and ion densities </li></ul><ul><li>Plasma density: N i =10 14 -10 20 m -3 </li></ul><ul><li>Particle temperature: </li></ul><ul><ul><li>Electrons:T e  1-8 eV </li></ul></ul><ul><ul><li>Ions: T i  1-3 eV </li></ul></ul><ul><li>Particle velocity: </li></ul><ul><ul><li>Electrons V e =10 5 -10 6 m/s </li></ul></ul><ul><ul><li>Ions V i =1-2  10 4 m/s </li></ul></ul>
  15. 15. COLD CATHODE – HOT CATHODE <ul><li>Momentarily cathode spot appearance on some location on the cathode surface – local temperature increase and local material melting and vaporization  erosion of cathode material </li></ul><ul><li>Generally, in conventional VAD, cathode (mainly, metallic material) is water cooled to eliminate gross melting of the cathode material and thermal damage of plasma source elements. The cathode is integrally cold – COLD CATHODE vacuum arc </li></ul><ul><li>Some materials (ex., semiconductors – Si ) – low conductivity at low temperature. Heating the cathode allows to increase material conductivity and sustain stable arc – HOT CATHODE vacuum arc (D. Arbilly et. al., EDPL-TAU, 1990s) </li></ul>
  16. 16. VACUUM ARC PLASMA JET <ul><li>VACUUM ARC PLASMA JET – metal plasma flow produced by the cathode spots – positively ionized particles of cathode material and electrons </li></ul><ul><li>IN CONTRAST TO PLASMA IN PECVD , ARE , or SPUTTERING (diode, triode, magnetron) – glow discharge plasma established in background gas (inert or reactive, or their mixture) – as usual , positive gas ions and electrons </li></ul>
  17. 17. MACROPATICLES IN PLASMA JET <ul><li>MACROPARTICLES , or micro-droplets , are molten droplets of cathode material, produced at erosion from the cathode spots, along with charged particles (electrons, ions) </li></ul><ul><li>Higher cathode melting temperature – lower plasma jet contamination by macroparticles (up to 99% ionized fraction for refractory metal plasmas) </li></ul><ul><li>Size: from 0.1  m up to ~100  m ( typical size in VAD processes – few  m) </li></ul><ul><li>Velocity: from 10 to ~800 m/s </li></ul><ul><li>Macroparticle content and size increase: </li></ul><ul><ul><li>With decreasing melting temperature of cathode material </li></ul></ul><ul><ul><li>With increasing arc current for certain cathode material </li></ul></ul>
  18. 18. MACROPARTICLES IN VAD COATINGS
  19. 19. CURRENT-VOLTAGE CHARACTERISTICS OF A DISCHARGE TUBE
  20. 20. COMPARISON OF PLASMAS IN VACUUM ARC DEPOSITION AND SPUTTERING SYSTEMS: High voltage (few hundreds V - ~1 kV), low current (~10 -2 -1 A) discharge High current (I arc =30-500 A), low voltage (10-50 V) discharge Electrical characteristics Bombardment by of target by plasma ions to eject particles (atoms/molecules) of material to be deposited Production of charged particle jet to be deposited on substrate Function of plasma Glow discharge induced in low pressure background working gas Metal (mainly) plasma, cathode erosion species Type of plasma SPUTTERING VACUUM ARC FEATURE
  21. 21. COMPARISON OF PLASMAS (CONTINUE) 10 14 -10 18 10 16 -10 20 Plasma density, m -3 (3-6)  10 3 (Ar ions) (1-2)  10 4 Ion velocity, m/s 10-40 50-150 Ion energy, eV No Yes Macroparticles SPUTTERING VACUUM ARC FEATURE
  22. 22. VACUUM ARC DEPOSITON HISTORY - I <ul><li>End XIXth century – A.W. Wright (mirror coatings), </li></ul><ul><li>T. Edison – layers for phonogram molds </li></ul><ul><li>Middle 1970s -1980 Kharkov, Soviet Union – new era in hard coating technology </li></ul><ul><li>1975-76 – Development of vacuum arc plasma source </li></ul><ul><li>1976-80 Development of vacuum arc deposition system and process of wear resistant coating deposition for cutting tools MoN, TiN, TiC </li></ul><ul><li>METHOD CIB (condensation – ion bombardment) </li></ul><ul><li>1978-80 Development of filtered vacuum arc deposition system </li></ul><ul><li>1980 – Development of Diamond-like coating (DLC) deposition process </li></ul><ul><li>1980 – Development and serial production of deposition systems (BULAT) </li></ul>
  23. 23. VACUUM ARC DEPOSITON HISTORY - II <ul><li>1979-1980 – patent and technology was acquired by Multi-Arc Vacuum Systems Inc. (NJ, USA) – wide-spread process use in the world </li></ul><ul><li>ION BOND  technique </li></ul><ul><li>Geography – Soviet Union (Former), USA, Canada, UK, Germany, France, Italy, Spain, China, Australia, etc. </li></ul><ul><li>Israel – from the beginning of 1990s (ISCAR) </li></ul><ul><li>Some leaders in R&D in vacuum arc deposition </li></ul><ul><ul><li>Lawrence Berkeley Laboratory, CA, USA </li></ul></ul><ul><ul><li>Kharkov National Research Center NIST, Ukraine </li></ul></ul><ul><ul><li>Balzers, Liechtenstein </li></ul></ul><ul><ul><li>CSIRO, Australia </li></ul></ul><ul><ul><li>EDPL, Tel-Aviv University, Israel </li></ul></ul><ul><ul><li>Etc., etc. </li></ul></ul>
  24. 24. VACUUM ARC DEPOSITON HISTORY - III <ul><li>Middle 1980s – Development of multi-layer and multi-component wear resistant coatings </li></ul><ul><li>1990s - Renaissance of filtered vacuum arc deposition </li></ul><ul><li>1990s – Industrial processes of DLC deposition </li></ul><ul><li>1990s – Super-hard nano-composite coatings (HV~50-60 GPa) </li></ul><ul><li>1990s – Development of ‘ion implantation – deposition’ processes </li></ul><ul><li>2000s – Development of deposition processes for superhard nanocomposite coatings </li></ul>
  25. 25. VACUMM ARC DEPOSITION Process Definitions, Terms <ul><li>Currently, the following definitions of conventional VAD may be found in literature (examples): </li></ul><ul><li>Vacuum Arc Evaporation /Deposition </li></ul><ul><li>Cathodic Vacuum Arc Deposition </li></ul><ul><li>Cathode Spot (Cathodic Spot) VAD </li></ul><ul><li>Cold Cathode VAD </li></ul><ul><li>Cathodic Vacuum Arc Ion Plating </li></ul><ul><li>Pulsed (or DC) VAD </li></ul><ul><li>Note: different non-proper definitions of VAD sometimes appear in literature, e.g.: </li></ul><ul><ul><li>ARC-PVD </li></ul></ul><ul><ul><li>Arc ion plating </li></ul></ul>
  26. 26. TYPICAL VACUUM ARC DEPOSITION (VAD) SYSTEM <ul><li>Water-cooled cylindrical cathode of conducting material </li></ul><ul><li>Water-cooled annular anode </li></ul><ul><li>Trigger mechanism </li></ul><ul><li>High current, low voltage DC power supply (dc welding power supply is suitable) </li></ul>VAD SYSTEM WITH CYLINDRICAL CATHODE AND ANNULAR ANODE
  27. 27. ARC INITIATION <ul><li>Arc ignition (triggering) – contact mode or breakdown mode </li></ul><ul><li>Contact mode - momentarily trigger electrode contact to the cathode surface – initial discharge </li></ul><ul><li>Initial cathode spots appear on the cathode surface </li></ul><ul><li>Erosion of cathode species from the cathode spots – electrons and positive ions of the cathode material – conducting medium in the inter-electrode space </li></ul><ul><li>Initial discharge initiates main discharge between the arc cathode and the anode </li></ul>
  28. 28. ARC OPERATION <ul><li>Arc ignited, cathode spots operate on the cathode surface </li></ul><ul><li>Part of electrons follows to anode to close electrical circuit – self-sustained arc operation </li></ul><ul><li>Plasma jet produced by the cathode spots passes via the anode aperture to the chamber and expands in the chamber. </li></ul><ul><li>Part of plasma flux arrives to the substrate forming a coating on its surface </li></ul><ul><li>Threshold arc current (I arc ) – minimum I arc at which arc may be ignited </li></ul>
  29. 29. CATHODE SPOTS <ul><li>Spots operating on the cathode surface produce flux of charged particles </li></ul><ul><li>Spots rapidly move on the cathode surface, disappear, appear re-appear, split, multiply </li></ul><ul><li>No spots on the cathode surface  no erosion of cathode species  no arc current flow  arc extinguishes </li></ul><ul><li>1 spot per 40-60 A I arc for most materials </li></ul><ul><li>Integrally cold cathode – good cathode cooling </li></ul>Tracks of cathode spot motion on Ti cathode surface, I arc ~100 A
  30. 30. ARC CURRENT (I arc ) AND ION CURRENT (I ion ) PRODUCED BY VACUUM ARC PLASMA SOURCE <ul><li>Larger I arc -> and larger number of spots: </li></ul><ul><ul><li>more stable arcing – i.e. less frequency of interruptions, extinguishing </li></ul></ul><ul><ul><li>larger ion flux produced by the cathode spots </li></ul></ul><ul><li>Ion current (I ion ) produced by vacuum arc plasma source: </li></ul><ul><li>I ion =(0.08-0.12)  I arc </li></ul><ul><li>However, larger I arc : </li></ul><ul><ul><li>larger arc power </li></ul></ul><ul><ul><li>larger heat flux produced by arc – additional requirements to cooling </li></ul></ul><ul><ul><li>larger ion flux contamination by macroparticles </li></ul></ul>
  31. 31. ARC POWER AND HEAT FLUX <ul><li>Arc power: P arc =I arc  V arc </li></ul><ul><li>V arc =15-50 V – arc voltage </li></ul><ul><li>In engineering calculations (especially, cooling system), rough estimation ~2/3P arc – heat flux to anode, and 1/3P arc – to cathode. </li></ul><ul><li>EXAMPLE </li></ul><ul><li>Ti arc operation*, I arc =300 A, V arc ~30 V </li></ul><ul><ul><li>PS : * Conditions typical for Ti arc in Deposition System 2 at Electrical Discharge and Plasma Lab., TAU </li></ul></ul><ul><li>P arc =9 kW, (anode ~6 kW, cathode ~3 kW) </li></ul><ul><li>Good cooling is needed taking into account that vacuum O-rings and electrical insulators may have temperature limit <100  C </li></ul>
  32. 32. COMMERCIAL VACUUM ARC DEPOSITION APPARATUS “Bulat-3” - NEW EPOCH IN CUTTING TOOL INDUSTRY (former USSR, 1981) Anode – water cooled vacuum chamber 3 independent plasma guns Chamber diameter ~ 60 cm
  33. 33. SCHEME OF COMMERCIAL VAD SYSTEM (“Bulat”- Type)
  34. 34. SCHEME OF COMMERCIAL TRIPLE CATHODE VAD SYSTEM
  35. 35. TRIPLE CATHODE VAD SYSTEM AT TAU
  36. 36. COATINGS TYPES THAT MAY BE FABRICATED USING VAD TECHNIQUES <ul><li>Coatings deposition of most conducting materials: pure metals, mixture of metals, carbon, solid solutions (e.g. Ti-Zr), inter-metallic compounds (e.g. Ni 3 Al), </li></ul><ul><li>Coatings of hard chemical compounds: nitrides, carbides, borides, oxides, etc – at deposition of metallic plasma in background of the reactive gas </li></ul><ul><li>Vide-range variation of metal:gas content in coating is possible </li></ul><ul><li>Super-hard diamond-like coatings at deposition of carbon plasma </li></ul><ul><li>Multi-component coatings – at simultaneous deposition of plasmas from different cathodes </li></ul><ul><li>Example : Ti and Zr plasmas in nitrogen background gas – coating (Ti,Zr)N </li></ul>
  37. 37. COATINGS TYPES THAT MAY BE FABRICATED USING VAD TECHNIQUES (Continue) <ul><li>Multi-layer coatings - at consequent deposition of plasmas from different cathodes </li></ul><ul><li>Example: bi-layer TiN-ZrN; three-layer </li></ul><ul><li>TiC-Ti(CN)-TiN, TiC-Al 2 O 3 -TiN </li></ul><ul><li>Multi-layer coatings with alternating “hard” layers Example: TiN-ZrN-TiN… </li></ul><ul><li>Super-hard (HV>50 GPa) multi-layer coating (up to 500 alternating layers, every few nm thick), forming super-lattice structure. Example: TiN-NbN-TiN… </li></ul><ul><li>Multi-layer coatings with alternating “hard” and “soft” layers. Example: TiN-Ti-TiN-Ti… </li></ul><ul><li>Graded coatings – with composition gradually changed across the coating thickness </li></ul>
  38. 38. REACTIVE VACUUM ARC DEPOSITION <ul><li>Reactive VAD – process of compound coating deposition as result of reaction of cathode spot produced plasma (metal plasma ions) with background reactive gas </li></ul><ul><li>Chemical reactions in plasma – </li></ul><ul><li>plasma-chemical reactions </li></ul><ul><li>Non-equilibrium reactions of highly activated plasma ions with atoms/molecules of background gas </li></ul><ul><li>Low-temperature reactions – possibility of refractory compound coating deposition at relatively low substrate temperatures (20-500  C) </li></ul><ul><li>REACTIVE VAD – EFFECTIVE PROCESS OF HARD (SUPER-HARD) WEAR RESISTANT COATING DEPOSITION AT LOW SUBTRATE TEMPERATURES - NITRIDES, CARBIDES, CARBO-NITRIDES, BORIDES, SILICIDES, etc. </li></ul>
  39. 39. REACTIVE VAD - HARD WEAR RESISTANT COATING DEPOSITION <ul><li>Since beginning of 1980s Reactive VAD is widely used for wear resistant coating deposition in cutting tool industry all over the world </li></ul><ul><li>Reactive VAD is well developed industrial process in different areas of coating application </li></ul><ul><li>Examples of industrial hard wear resistant coatings: nitrides TiN, ZrN, (Ti,Al)N, carbides TiC, ZrC , carbo-nitrides Ti(C,N), borides TiB 2 </li></ul><ul><li>Metal vapor plasma: Ti, Zr, Hf, Nb, Ta, Cr, Mo, Al </li></ul><ul><li>Reactive gases: nitrides – N 2 , carbides – CH 4 or C 2 H 2 , carbo-nitrides – C 2 H 2 +N 2 , oxides – O 2 , silicides – SiH 4 </li></ul>
  40. 40. REACTIVE VAD OF TiN COATING Model by M.Sakaki & T.Sakakibara, IEEE. Trans. Plasma Sci., vol. 22 (1994), p. 1049-1054 <ul><li>Cathode – Ti </li></ul><ul><li>Reactive gas N 2 </li></ul><ul><li>On the substrate surface </li></ul><ul><li>N 2 + dissociates to N or N + at collisions with Ti + </li></ul><ul><li>Ti+N  TiN (  H=-338 kJ/mol) </li></ul><ul><li>Spectroscopic studies: </li></ul><ul><li>Molecules N 2 are excited and ionized by collisions with high energy electrons  N 2 + molecular ions </li></ul><ul><li>No N or N+ in the plasma flux </li></ul><ul><li>Ti 2+ emitted from the cathode are recombined with electrons to Ti + and Ti </li></ul><ul><li>No TiN in the plasma flux was found </li></ul><ul><li>TiN formation - on the substrate surface as result of exothermic chemical reaction </li></ul>
  41. 41. ION PLATING – SUBSTRATE BOMBARDMENT BY POSITIVE PLAMA IONS ION PLATING IN VAD Role of negative bias voltage on surface bombardment by plasma ions and coating deposition process
  42. 42. NEGATIVE BIAS VOLTAGE APPLIED TO THE SUBSTRATE IN VAD PROCESS Negative bias voltage to the substrate relative the grounded anode Substrate ion bombardment by positive metal ions
  43. 43. COATING DEPOSITION RATE vs BIAS VOLTAGE TO THE SUBSTRATE <ul><li>Low negative bias voltage to the substrate |-V bias | – coating deposition </li></ul><ul><li>High |V bias | - sputtering of atoms/molecus from the substrate surface </li></ul>
  44. 44. BIAS VOLTAGE APPLICATION: ACTION OF HIGH |V bias | <ul><li>HIGH NEGATIVE V bias – SPUTTERING: </li></ul><ul><li>Effective substrate surface sputter ion cleaning within deposition chamber (typically, at |Vbias|  1000 V) </li></ul><ul><li>Surface activation before coating deposition - good coating adhesion </li></ul><ul><li>Positive ion impingement at further increase |V bias | (>10 kV) - may cause ion implantation to the substrate surface </li></ul><ul><li>Ion bombardment at large |V bias | - effective surface heating – in many conventional VAD systems and processes - no necessity in additional substrate heating source </li></ul>
  45. 45. BIAS VOLTAGE APPLICATION: ACTION OF LOW |V bias | <ul><li>LOW |V bias | - COATING DEPOSITION </li></ul><ul><li>Nucleation stage – increased nucleation site density </li></ul><ul><li>Interface formation – additional thermal energy to the surface, activation diffusion and surface reactions </li></ul><ul><li>Ion bombardment during deposition: </li></ul><ul><li>(1) densification of growing film </li></ul><ul><li>(2) removal (sputtering) of unbonded and ‘weakly bonded’ species from the coating </li></ul><ul><li>(3) adding thermal energy </li></ul><ul><li>(4) activation ad-atom/ion mobility </li></ul><ul><li>(5) activation plasma-chemical reactions on the surface </li></ul>
  46. 46. INFLUENCE OF BIAS VOLTAGE ON COATING STRUCTURE AND PROPERTIES <ul><li>Bias voltage determines: </li></ul><ul><li>Structural characteristics of the coating </li></ul><ul><ul><li>crystalline structure </li></ul></ul><ul><ul><li>preferred orientation (texture) </li></ul></ul><ul><ul><li>grain size </li></ul></ul><ul><li>Deposition rate </li></ul><ul><li>Physical and mechanical properties </li></ul><ul><li>Service properties </li></ul>
  47. 47. BIAS VOLTAGE: CONVENTIONAL COATING DEPOSITION PROCESS <ul><li>CONVENTIONAL DEPOSITION PROCESS, AS A RULE, CONSISTS OF TWO STAGES WITH RESPECT OF BIAS APPLICATION </li></ul><ul><li>Application of high |V bias | - ion cleaning and substrate heating up to required temperature ( ex., |V bias |  1000 V for Ti plasma) </li></ul><ul><li>Coating deposition at lower |V bias | to fabricate the coating with required structure and properties (ex., |V bias |=20-300 V for Ti) </li></ul>
  48. 48. POSSIBLE NEGATIVE EFFECTS OF BIAS APPLICATION TO THE SUBSTRATE <ul><li>Appearance of spots on the negatively biased substrate , which may momentarily serve as a cathode </li></ul><ul><ul><li>Spot traces on the surface – coating damage </li></ul></ul><ul><ul><li>To eliminate spots on the substrate – use power supply equipped with micro-arc inhibiting circuits is preferable </li></ul></ul><ul><li>Intensive heating by high energy plasma flux may cause overheating and damage of substrate material, especially, small coated workpieces, low-size cutting tools from HSS steel (danger of annealing), sharp, low-radius parts of a substrate, etc. </li></ul><ul><ul><li>Accurate evaluation of heat flux </li></ul></ul><ul><ul><li>Good temperature control </li></ul></ul>
  49. 49. CALCULATION OF HEAT FLUX TO THE SUBSTRATE <ul><li>Heat flux to the substrate, Q </li></ul><ul><li>Q=J i  (|V bias |+E i ) </li></ul><ul><li>J i - ion current density to the substrate </li></ul><ul><li>E i =E pi +E ki +E vi – total ion energy </li></ul><ul><ul><li>E pi – ionization potential (Ti~15 eV, Zr ~19 eV) </li></ul></ul><ul><ul><li>E ki – kinetic energy (Ti – 41.9 eV, Zr -57.3 eV) </li></ul></ul><ul><ul><li>E vi – heat of vaporization (Ti - 4.8 eV/atom, Zr – 6.2 eV/atom) </li></ul></ul><ul><li>E pi , E ki , E vi – handbook or reference data for different ions </li></ul><ul><li>Ion cleaning (sputtering) stage: |V bias I>>E i  substrate heating mainly due to bias voltage </li></ul><ul><li>Deposition stage: V bias ~20-300V  substrate heating due to both bias voltage and ion energy </li></ul>
  50. 50. DEBYE LENGTH <ul><li>Penetration depth of electric field produced by bias voltage applied to the substrate – </li></ul><ul><li>Debye length  D </li></ul><ul><li> D =(  0 kT e /2e 2 n e ) 1/2 </li></ul><ul><li> D is the distance from the substrate, where potential drops from the substrate to the plasma potential </li></ul><ul><li>EXAMPLE : </li></ul><ul><li>Determine  D for vacuum arc plasma: </li></ul><ul><ul><li>electron density n e =10 18 m -3 </li></ul></ul><ul><ul><li>electron temperature T=3  10 4 K </li></ul></ul><ul><li> 0 =8.85  10 -12 F/m – dielectric permeability </li></ul><ul><li>k=1.38  10 -23 J/K – Boltzmann constant </li></ul><ul><li>e=1.6  10 -19 C – electron charge </li></ul><ul><li> D =(  0 kT e /2e 2 n e ) 1/2 =8.5  10 -6 m=8.5  m </li></ul>
  51. 51. ION FLUX TO THE SUBSTRATE IN VAD - EFFECTIVENESS OF DEPOSITION PROCESS <ul><li>Measurement of saturated ion current collected by probe </li></ul><ul><li>Langmuir probe </li></ul><ul><li>Ion current and coating deposition rate </li></ul>
  52. 52. MEASUREMENT OF ION FLUX TO THE SUBSTRATE (ION CURRENT) Negatively biased Langmuir probe – saturation ion current measurement Saturation ion current characterizes deposition system effectiveness
  53. 53. VOLTAGE-CURRENT CHARACTERISTICS OF LANGMUIR PROBE Evaluation of deposition rate – saturated ion current is important
  54. 54. SATURATION ION CURRENT AND DEPOSITION RATE <ul><li>Evaluation of coating deposition rate v d </li></ul>J i – ion current density m i – ion mass z – average ion charge number e – electron charge  - density  =0.8-0.9 – sticking coefficient
  55. 55. ION CURRENT AND DEPOSITION RATE <ul><li>For VAD systems v d  1…60 nm/s </li></ul><ul><li>Ion current and deposition rate: </li></ul><ul><li>Generally increase with increasing arc current </li></ul><ul><li>Decrease with increasing distance from the plasma gun to the substrate </li></ul><ul><li>Generally decrease, or first increase, pass maximum and then rapidly decrease with increasing background gas pressure </li></ul>
  56. 56. MAGNETIC FIELD IN VACUUM ARC DEPOSITION <ul><li>Cathode spot motion in the magnetic field </li></ul><ul><li>Magnetic guiding of the plasma jet </li></ul><ul><li>Plasma transport in straight and curvilinear plasma duct </li></ul><ul><li>Macroparticle filtering </li></ul>
  57. 57. MAGNETIC FIELD IN VACUUM ARC PLASMA DEPOSITION SYSTEMS EDPL, TAU – System 3
  58. 58. MAIN FUNCTIONS OF MAGNETIC FIELD IN VAD SYSTEMS <ul><li>MAGNETIC FIELD IN THE VICINITY OF THE CATHODE – to confine (stabilize) the cathode spot motion on the cathode surface </li></ul><ul><li>The spots may move out of the cathode surface to the cathode holder, causing unstable arcing </li></ul><ul><li>MAGNETIC FIELD APPLIED TO THE PLASMA DUCT – to guide the plasma flux from the cathode of the plasma gun (plasma source) to the substrate </li></ul><ul><li>Application of an axial magnetic field to guide plasma– significant increase of the plasma flux to the substrate and coating deposition rate </li></ul>
  59. 59. CATHODE SPOT MOTION ON THE FACE SURFACE OF CYLINDRICAL CATHODE. NO MAGNETC FIELD <ul><li>Generally, random spot motion on the cathode face surface if no magnetic field is applied </li></ul><ul><li>Random = spontaneous, chaotic </li></ul><ul><li>Sometimes, the spots tend to move to the cathode side </li></ul><ul><ul><li>unstable arc operation, arc extinguishing </li></ul></ul><ul><ul><li>contamination of plasma flux and coating </li></ul></ul><ul><ul><li>Continuous spot operation on the side may cause damage of the plasma gun </li></ul></ul>Random spot motion on cathode surface
  60. 60. MAGNETIC FIELD EFFECT ON THE CATHODE SPOT MOTION <ul><li>TWO EFFECTS OF MAGNETIC FIELD </li></ul><ul><li>ON THE CATHODE SPOT MOTION: </li></ul><ul><li>Spot movement in retrograde –(E  B) direction (circular motion on the cathode surface) </li></ul><ul><li>Spot motion towards acute angle opening between the magnetic field line and its projection on the cathode surface – acute angle rule </li></ul>
  61. 61. 1. RETROGRADE (-E x B) CATHODE SPOT MOTION IN MAGNETIC FIELD <ul><li>Round cathode, annular anode, axial magnetic field </li></ul><ul><li>Retrograde (Anti-Amperic) spot motion </li></ul><ul><li>Spot velocity vector: v =- E x B , or v =- j x B </li></ul><ul><li>Note : Amperic direction : +E x B </li></ul><ul><li>Round symmetry – superposition of random motion  preferential rotation of spots on the cathode surface </li></ul><ul><li>Opposite field B direction – opposite direction of rotation </li></ul>
  62. 62. 2. “ACUTE ANGLE” RULE. CYLINDRICAL CATHODE - TWO CASES OF MAGNETIC FIELD CONFIGURATION <ul><li>Left – Magnetic field, guiding the plasma from the cathode spots - acute angle  is directed outwards (towards cathode side), the spots move outside the cathode surface – unstable arcing </li></ul><ul><li>Right – “arched” field produced by the CATHODE COIL – circumferential spot motion on the cathode surface – stable arcing </li></ul>Spots move outside the cathode surface – unstable arcing “ Arched field” – spot motion on circumferential trajectory on cathode
  63. 63. EFFECT OF MAGNETIC FIELD ON CATHODE SPOT MOTION <ul><li>“ Acute angle” rule – the spots tend to move towards the acute angle (  ) opening between the magnetic field line and its projection on the cathode surface </li></ul><ul><li>Simultaneous action of the retrograde (-ExB) motion and ‘acute angle” rule </li></ul><ul><li> directed outside the cathode surface – circumferential spot motion + tendency of spots to move outwards - to the cathode side – unstable arcing, interruption, extinguishing of the arc </li></ul><ul><li>“ Arched” field on the cathode surface – circumferential spot motion on the defined trajectory on the cathode surface </li></ul>
  64. 64. “ ARCHED FIELD” - CIRCUMFERENTIAL SPOT MOTION ON DEFINED TRAJECTORY <ul><li>“ Arched” field on the cathode surface is produced by two coils connected in opposite direction: Cathode coil (B r =12 mT) and Spacer coil, B=-4 mT </li></ul><ul><li>Ti arc, I arc ~160 A, System 2, EDPL </li></ul>Magnetic field plot in the cathode vicinity Cathode erosion track V. Zhitomirsky et al., 1994
  65. 65. SPOT MOTION ON ROUND CATHODE SURFACE (Ti arc, System 2) No magnetic field – random spot motion Acute angle directed outwards – Spot operation on the cathode side “ Arched” field – circumferential spot motion V. Zhitomirsky, JVST, 1995
  66. 66. “ ARCHED FIELD” CYLINDRICAL CATHODE <ul><li>“ Arched magnetic field is produced by magnetic coil positioned in water-cooled cavity - the opposite side from the cathode surface </li></ul>Water-cooled cavity Cathode surface Cathodes
  67. 67. “ ARCHED” FIELD - RECTANGULAR CATHODE DESIGH Magnetic field configuration: QuickField TM plot Cathode spot Motion track Rectangular cathode, System 4, EDPL
  68. 68. EFFECT OF “ACUTE ANGLE” IN CONE CATHODE DESIGN – SPOT CONFINEMENT IN CATHODE CENTER Cathode Anode Focusing Coil Guiding Coil Acute angle  - between the magnetic line and cathode cone surface – spot motion towards the cathode center Axial magnetic field – spot confinement in the cathode center Cone cathode in axial field Cathode Coil
  69. 69. MAGNETIC PLASMA GUIDING TO THE SUBSTRATE IN THE STRAIGHT CYLINDRICAL DUCT Axial magnetic field produced by coils 2-4 positioned co-axially with duct axis - significant increase in plasma flux to the substrate System 3, EDPL
  70. 70. PLOT OF MAGNETIC FIELD LINES PRODUCED BY AXIAL COILS (Axial Cross-Section) <ul><li>QuickField  plot – axisymmetric cross-section </li></ul><ul><li>Magnetic field lines parallel to the duct wall </li></ul><ul><li>Relatively weak field (B=10-30 mT) may be successfully used to guide the plasma jet in the straight cylindrical duct </li></ul>Substrate Cathode
  71. 71. PRINCIPLES OF PLASMA GUIDING IN THE MAGNETIC FIELD PLASMA TRANSPORT IN THE STRAIGHT AND CURVILINEAR DUCT
  72. 72. MOTION OF A CHARGED PARTICLE IN MAGNETIC FIELD <ul><li>Motion of an individual charged particle in magnetic field – two velocity components: </li></ul><ul><li>v || - velocity component in direction parallel to the magnetic field lines </li></ul><ul><li>v  - velocity component in normal direction – normal drift </li></ul><ul><li>Resulting particle motion – gyration , cycloid motion </li></ul><ul><li>Radius of particle gyration - Larmor radius </li></ul>
  73. 73. LARMOR RADIUS OF PARTICLE GYRATION <ul><li>Larmor radius, r L : </li></ul><ul><li>r L =v  /  L =v  m/qB </li></ul><ul><li> L - cyclotron frequency,  L =qB/m </li></ul><ul><li>v  - velocity normal to B </li></ul><ul><li>m – particle mass </li></ul><ul><li>q – particle charge </li></ul><ul><li>r L low (electrons) – particles are magnetized , their motion is confined by magnetic field lines </li></ul><ul><li>Criterion of particle magnetization : r L <<D/2 </li></ul><ul><ul><li>where D – duct diameter </li></ul></ul><ul><li>r L smaller and magnetization stronger with larger magnetic field B and smaller particle mass m </li></ul>
  74. 74. GUIDING OF A PLASMA JET IN AN AXIAL MAGNETIC FIELD <ul><li>TWO PRINCIPLES OF PLASMA GUIDING: </li></ul><ul><li>Guiding of plasma by a strong magnetic field (B>0.1T) – electrons and ions are magnetized </li></ul><ul><li>Guiding by a relatively weak magnetic field – electrons are magnetized, while ions are non-magnetized </li></ul><ul><li>STRONG MAGNETIC FILELD </li></ul><ul><li>Electrons and ions of plasma are magnetized, and their motion is confined on magnetic field lines </li></ul><ul><li>High transport efficiency, low particle losses in the way to the substrate </li></ul><ul><li>However, strong magnetic field may cause instability: </li></ul><ul><li>Influence on the cathode region –unstable cathode spot operation </li></ul><ul><li>Cut-off magnetized electron flow from the anode </li></ul><ul><li>Difficulties to produce strong field, expensive power supply </li></ul>
  75. 75. GUIDING OF A PLASMA JET IN A RELATIVELY WEAK MAGNETIC FIELD <ul><li>In a relatively weak magnetic field (10-30 mT) – electrons are magnetized while ions are non-magnetized , or only partly magnetized </li></ul><ul><li>Electron motion is confined by magnetic field lines, whereas ions are guided in the direction magnetic field by electrical field produced by magnetized electrons – quasi-neutral plasma </li></ul><ul><li>Lower effectiveness of ion guiding than in the case of full particle magnetization (ions and electrons) </li></ul><ul><li>However, relatively weak field - more frequent use in deposition systems than strong filed </li></ul><ul><li>Lower probability of unstable arcing, while electron cut-off from the anode is still possible </li></ul><ul><li>No necessity in specific power supply and water-cooled magnetic coils in most cases </li></ul>
  76. 76. PARTICLE MAGNETIZATION EXAMPLE <ul><ul><li>Given: axial field B=12 mT. Duct radius – R=8 cm. </li></ul></ul><ul><li>Question: Are the plasma particles (electrons and ions) for Al and Sn plasmas magnetized or non-magnetized? </li></ul><ul><li>Electrons: r Le =v  e m e /eB </li></ul><ul><ul><li>v  e ~6.7  10 5 m/s (T e =3 eV) </li></ul></ul><ul><ul><li>m e ~9.1  10 -31 kg </li></ul></ul><ul><ul><li>e=1.6.10 -19 C </li></ul></ul><ul><li>r Le  3.10 -4 m=300  m<<R </li></ul><ul><li>Ions: r Li =v  i m i /ZeB </li></ul><ul><ul><li>v  Al ~1.5.10 3 m/s, v  Sn ~4.10 3 m/s </li></ul></ul><ul><ul><li>m Al =27  1.6  10 -27 kg, m Sn =119  1.6  10 -27 kg </li></ul></ul><ul><ul><li>Z Al =1.7, Z Sn =1.5 </li></ul></ul><ul><ul><li>(r L ) Al =2 cm<R , (r L ) Sn =26.4 cm >R </li></ul></ul><ul><li>Conclusion: </li></ul><ul><li>(1) Al plasma – electrons and ions are magnetized </li></ul><ul><li>(2) Sn plasma - electrons are magnetized while Sn ions are non-magnetized </li></ul>
  77. 77. ELECTRON CUT-OFF FROM THE ANODE IN MAGNETIC FIELD – UNSTABLE ARCING <ul><li>Annular anode </li></ul><ul><li>Axial magnetic field B </li></ul><ul><li>Electrons are magnetized on the magnetic field lines – radial drift is suppressed </li></ul><ul><li>Increasing B – electron magnetization – electron cut-off from the anode , unstable arcing </li></ul><ul><li>Full electron magnetization - electrons cannot arrive the anode – arc extinguishes </li></ul><ul><li>For stable arc – design magnetic field in the vicinity of the plasma gun allowing the electrons to achieve the anode – close electrical circuit </li></ul>B
  78. 78. SPATIAL DISTRIBUTION OF THE PLASMA BEAM IN MAGNETIC FIELD <ul><li>Gaussian fitting: Ion current density J=J o exp(-r 2 /  2 ) </li></ul><ul><ul><li>J 0 – maximum current density </li></ul></ul><ul><ul><li>r - radial coordinate </li></ul></ul><ul><ul><li> - distribution width </li></ul></ul><ul><li>Stronger guiding magnetic field – lower  (lower beam width) </li></ul><ul><li>Steering field normal to the plasma beam – plasma beam swiping across the substrate – more uniform coating thickness </li></ul>Spatial distribution of ion current density Multi-probe for distribution measurement V. Zhitomirsky et al . Surf. Coat. Technol. 1995
  79. 79. FILTERED VACUUM ARC DEPOSITION (FVAD) <ul><li>Along with charged particles, the plasma flux produced by the cathode spots contains some amount of macroparticles </li></ul><ul><li>Lower melting point of the cathode material – stronger flux contamination by macroparticles </li></ul><ul><li>Coating contamination by macroparticles leads to significant decrease in coating quality and service properties, especially in electronics applications </li></ul><ul><li>One of the ways to remove macroparticles or significantly reduce their concentration is a magnetic filtering - plasma transport via a curvilinear magnetic duct </li></ul><ul><li>A QUARTER TORUS MACROPARTICLE FILTER is generally used for macroparticle filtering from the plasma flow </li></ul>
  80. 80. VAD SYSTEM WITH A QUARTER -TORUS MAGNETIC FILTER Plasma flux is guided by toroidal magnetic field to the substrate positioned in the chamber, while macroparticles are fully or mainly removed from the flux System 2, EDPL
  81. 81. SCHEME OF FILTERED VAD (FVAD) SYSTEM (System 2 at TAU)
  82. 82. MAGNETIC FIELD CONFIGURATION IN THE TOROIDAL DUCT <ul><li>No direct line-of sight from the cathode to the substrate via toroidal duct </li></ul><ul><li>Toroidal duct and chamber - 160 mm diameter </li></ul><ul><li>Toroidal field B T =16-28 mT (Ti, Sn plasmas) </li></ul><ul><li>Increasing B T >28 mT –cathode spot operation on the cathode side -unstable arc operation </li></ul>
  83. 83. MACROPARTICLE FILTERING: VAD SYSTEM WITH A QUARTER TORUS FILTER <ul><li>Magnetic field generally parallel to the duct wall </li></ul><ul><li>Plasma (charged particles) is transported via the duct by the action of applied toroidal field (magnetized, or non-magnetized ions), while macroparticles are not affected by the magnetic field </li></ul><ul><li>Macroparticles mainly leave the plasma flow and remain on the duct wall </li></ul><ul><li>DISADVANTAGES OF A QUARTER TORUS FILTER </li></ul><ul><li>Low efficiency of plasma transport via the toroidal duct (typically, <20%) – low deposition efficiency </li></ul><ul><li>Part of macroparticles may reflect from the duct wall and return to the plasma jet </li></ul>
  84. 84. COATINGS DEPOSITED WITH UNFILTERED AND FILTERED VAD Macroparticles in Coating Unfiltered VAD Macroparticle-Free Coating Filtered VAD
  85. 85. “ S-FILTER” (A. Anders, Berkeley, USA) <ul><li>Better macroparticle removal on expense of lower deposition process efficiency in comparison with a quarter-torus filter </li></ul>
  86. 86. “ OPEN” S-FILTER DESIGN (A. Anders, Berkeley, USA) TOROIDAL FILTER WITH WALL – part of small macroparticles may reflect from the filter wall and follow to the substrate with plasma jet “ Open” S-filter (Anders’ design) – macroparticles mainly exit between windings and leave plasma jet CATHODE To substrate <ul><li>Magnetic coil windings – </li></ul><ul><li>water cooled pipe </li></ul><ul><li>Filtered source is </li></ul><ul><li>situated within vacuum </li></ul><ul><li>chamber </li></ul><ul><li>No wall –no MP reflection </li></ul>
  87. 87. ADVANTAGES AND DISANVANTAGES OF MACROPARTICLE FILTERING <ul><li>ADVANTAGES </li></ul><ul><li>Macroparticle removal. </li></ul><ul><li>Plasma filtering - an only possibility of high quality deposition of transparent conducting (SnO 2 , ZnO), Diamond like, Si coatings by VAD </li></ul><ul><li>DISADVANTAGES </li></ul><ul><li>Low plasma throughput, high losses in the filter duct – low productivity. 70-90% of plasma flux may be lost in the filter. </li></ul><ul><li>Complicated process parameter control </li></ul><ul><li>Narrow plasma beam exited from the toroidal duct </li></ul>
  88. 88. DUAL FILTERED VACUUM ARC PLASMA SOURCE <ul><li>Dual FVAD source – increased deposition effectiveness </li></ul><ul><li>Possible deposition of multi-component and multi-layer coatings </li></ul>From: P. Martin, A. Bendavid, Thin Solid Films , 2001 More about FVAD sources – see R.L. Boxman and V.N. Zhitomirsky, “ Vacuum Arc Deposition Devices”, Rev. Sci. Instrum. 77 021101 (2006)
  89. 89. LARGE-AREA RECTANGULAR FVAD SYSTEM (System 4, EDPL) <ul><li>Conceptual application of round design to rectangular system </li></ul><ul><li>Macroparticle filter – two 45  bends </li></ul><ul><li>Flat samples 42x42 cm, substrate transport through the filter outlet </li></ul>
  90. 90. SCHEME OF RECTANGULAR FVAD SYSTEM AND CATHODE APPLICATION: Transparent conducting SnO 2 coating deposition on flat substrates Rectangular cathode with cathode spot trace System 4, EDPL
  91. 91. HOT REFRACTORY ANODE VACUUM ARC (HRAVA) <ul><li>HRAVA-effective method of macroparticle-free coating fabrication </li></ul><ul><li>Hot refractory anode – C, Mo, W </li></ul><ul><li>Radial plasma streams </li></ul>Courtesy of Prof. I. Beilis, EDPL-TAU Substrate Substrate Anode region Cathode region Plasma flux from:
  92. 92. CATHODE SPOT MOTION ON A LONG CYLINDRICAL CATHODE <ul><li>“ Heliclal Arc” </li></ul><ul><li>Cathode spot motion on a long rod cathode </li></ul><ul><li>Magnetic field is applied by a coil co-axial with the cathode axis </li></ul><ul><li>E. Pinchasov, 1985-1989 </li></ul>
  93. 93. COMPACT VAD SOURCE WITH CONE CATHODE, TAU, 2004-2008
  94. 94. VACUUM ARC PLASMA EVAPORATION (VAPE) EXAMPLE: NOT CATHODIC VAD! <ul><li>Paper: T. Minami, S. Ida and T. Miyata, “High rate deposition of transparent conducting oxide thin films by vacuum arc plasma evaporation ”, Thin Solid Films 416 (2002) 92–96 </li></ul>Vacuum Arc Plasma Evaporation (VAPE) – not cathode spot VAD! Arc plasma of gas (Ar+O 2 ) Gas pressure – 0.075-1 Pa (5  10 -4 -7.5  10 -3 Torr) Power – 2.5 10 kW Evaporated material – sintered from powders ZnO and Ga 2 O 3 Deposition of Ga-Zn-oxide (GZO) coating
  95. 95. LITERATURE <ul><li>R.L. Boxman, D. Sanders and P. Martin, Handbook of Vacuum Arc Science and Technology, Noyes Publications, Park Ridge, NJ, 1995. </li></ul><ul><li>R.L. Boxman and V.N. Zhitomirsky, “Vacuum arc deposition devices” Rev. Sci. Instrum. , 77 , No. 2 (2006), 021101. </li></ul>
  96. 96. Multi-Source VAD Systems Hybrid Deposition Systems
  97. 97. MULTI-SOURCE VAD SYSTEMS <ul><li>Industrial VAD systems are often equipped with few cathode spot plasma sources. This allows: </li></ul><ul><ul><li>Deposition of multi-component and multi-layer coatings when the plasma of different cathode materials should be deposited on the substrate </li></ul></ul><ul><ul><li>Increase deposition efficiency (rate) - the cathodes of the same material may be used in different sources of the VAD system </li></ul></ul><ul><li>Modern VAD systems - few filtered VAD sources </li></ul>
  98. 98. INDUSTRIAL SYSTEM WITH DUAL FILTERED VAD SOURCE V. Gorokhovsky, SCT 2004
  99. 99. COMBINED (HYBRID) DEPOSITION SYSTEMS <ul><li>Different deposition processes have their advantages and disadvantages </li></ul><ul><li>Modern tendency – use different deposition processes in one system - HYBRID deposition systems </li></ul><ul><li>Hybrid (combined) system may include different deposition sources, for example: </li></ul><ul><ul><li>VAD+E-Beam </li></ul></ul><ul><ul><li>VAD+ Magnetron Sputtering </li></ul></ul><ul><ul><li>VAD+PECVD </li></ul></ul><ul><ul><li>VAD+ E-Beam + Magnetron Sputtering + PECVD </li></ul></ul>
  100. 100. HYBRID DEPOSITION SYSTEM BY V. GOROHOVSKY V. Gorokhovsky et. Al., SCT 1993 Substrate Filtered VAD Source E-Beam Source
  101. 101. LARGE HYBRID SYSTEM WITH VAD AND MAGNETRON SPUTTERING SOURCES CemeCon AG (Germany) By the courtesy of Dr. Christoph Shiffers (CemeCon AG), March 2009
  102. 102. HYBRID VAD-CVD SYSTEM FOR SUPERHARD COATINGS nc -MeN/ a -Si 3 N 4 <ul><li>NOVEL SUPERHARD NANOCOMPOSITE COATINGS </li></ul><ul><li>HV>40 GPa </li></ul><ul><li>nc -MeN/a-Si 3 N 4 </li></ul><ul><li>Me – refractory metal (Ti, Zr, Hf, V, Vb, Ta, Cr), or Me+Al </li></ul><ul><li>Nanoscale crystalline (nc) nitride is dissolved in amorphous (a) Si 3 N 4 matrix </li></ul><ul><li>Simultaneous deposition of MeN (VAD source) and Si 3 N 4 (CVD source) </li></ul><ul><li>Example: </li></ul><ul><li>nc -TiN/a-Si 3 N 4 </li></ul>P. Martin et al., SCT 2004
  103. 103. APPENDIX 1 Pre-Deposition Substrate Ion Cleaning and Ion Plating in Deposition Processes * <ul><li>Pre-Deposition Substrate Ion Cleaning </li></ul><ul><li>Ion Plating </li></ul><ul><li>* May be useful for home tasks and examine preparation </li></ul>
  104. 104. ION BOMBARDMENT IN PVD PROCESSES <ul><li>It was shown (Lectures 5, 6), that substrate surface and growing coating bombardment by positive plasma ions plays an important role in the coating properties </li></ul><ul><li>Substrate ion bombardment - sputter ion cleaning (ion etching) of the substrate surface prior to coating deposition in PVD processes – effective surface preparation  increased coating adhesion to the substrate </li></ul><ul><li>Ion bombardment of the growing coating (ion plating) – dense structure, enhanced coating properties </li></ul>
  105. 105. SPUTTER ION CLEANING IN GLOW DISCHARGE PLASMA <ul><li>Sputter ion cleaning of the substrate surface in glow discharge plasma may be realized in almost every PVD process studied in the present course, and even in CVD in which the plasma is used, i.e., PACVD/PECVD </li></ul><ul><li>Negative bias voltage (~- 1 kV- few kV) is applied to the substrate accelerate positive gas ions of the plasma towards the substrate </li></ul><ul><li>PVD processes: In all sputtering processes, and reactive evaporation (ARE) processes – glow discharge plasma is already used </li></ul><ul><ul><li>As the source of glow discharge plasma already exists, substrate sputter ion cleaning can be easily realized </li></ul></ul><ul><li>Systems with crossed plasma source and deposition system (ARE with resistively heated source, or Cathodic sputtering– triode system) shot-time ion cleaning: </li></ul><ul><ul><li>Glow discharge plasma of inert gas, or reactive gas (rarely) </li></ul></ul><ul><ul><li>Negative bias voltage applied to the substrate (~-1-1.5 kV) </li></ul></ul><ul><ul><li>Source produced vapor species to be coated is not activated during ion cleaning </li></ul></ul>
  106. 106. CATHODIC SPUTTERING: DC TRIODE SYSTEM Negative bias voltage to the substrate – Sputter ion cleaning prior to deposition Negative bias voltage to the target – Sputter deposition
  107. 107. SPUTTER ION CLEANING IN GLOW DISCHARGE PLASMA <ul><li>Diode cathodic sputtering system: </li></ul><ul><ul><li>Short-term connections of target and substrate in the reverse order ( substrate as cathode ), if the target and the substrate are the electrodes for glow discharge plasma </li></ul></ul><ul><ul><li>Short-term connection of substrate as the cathode , if glow discharge at deposition is sustained between the target (cathode) and the chamber wall </li></ul></ul><ul><li>Conventional thermal evaporation system with resistively heated source – additional electrode and gas inlet (mainly, inert) to ignite glow discharge between the substrate ( connected as the cathode ) and additional electrode </li></ul><ul><li>High energy of bombarding ions </li></ul><ul><li>E=J ion  S  |V bias | </li></ul><ul><ul><li>J ion – bombarded ion flux </li></ul></ul><ul><ul><li>S – bombarded area </li></ul></ul>
  108. 108. ION BOMBARDMENT OF COATING DURING DEPOSITION <ul><li>Positive role of ion bombardment of growing film – ION PLATING - was firstly studied and realized in PVD (thermal evaporation, E-Beam evaporation) systems by D. Mattox in 1960s-1970s (see Lecture 5) </li></ul><ul><li>Generally, ION PLATING may be realized in most PVD systems where the glow discharge plasma is used , i.e. ARE, Sputter deposition, as well as in PECVD/PACVD, if a negative bias voltage , separated from the plasma source, may be applied to the substrate </li></ul><ul><li>Generally, energy of bombarding gas ions at ION PLATING is lower than that at SPUTTER ION CLEANING , and the bias voltage at ion plating usually is lower </li></ul>
  109. 109. ION BOMBARDMENT IN VACUUM ARC DEPOSION <ul><li>In VAD processes highly ionized metal plasma flow is produced at erosion from the cathode spots – no additional source of plasma is necessary to realize sputter ion cleaning and ion plating </li></ul><ul><li>In VAD, the substrate surface and growing film are bombarded by metal ions having much higher energy than gas ions of glow discharge plasma </li></ul><ul><li>Conventional (industrial) VAD process – 2 stages </li></ul><ul><ul><li>Substrate sputter ion cleaning and heating - at high negative bias voltage (typically, |V bias |=1-1.5 kV for Ti ions) </li></ul></ul><ul><ul><li>Deposition process – lower negative bias voltage to the substrate – few tens – few hundreds Volts </li></ul></ul>
  110. 110. ION BOMBARDMENT IN VACUUM ARC DEPOSION <ul><li>In VAD processes – effective sputter ion cleaning in vacuum , in contrast to other PVD processes where the background gas is necessary to sustain a glow discharge plasma </li></ul><ul><li>In VAD the growth of deposited coating is under bombardment of energetic metal ions, even if no bias voltage is applied to the substrate (self bias) </li></ul><ul><ul><li>Energy of bombarding metal ions is quite enough to influence on structure and properties of growing film </li></ul></ul><ul><li>First industrial VAD process developed in the former USSR at the end of 1970s was named Condensation – Ion Bombardment (CIB), considering that coating condensation process (coating growth) takes place under ion bombardment = ION PLATING </li></ul>

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