Glow Discharge

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Glow Discharge

• Glow-discharge sputtering means the energetic
  particles used to strike target is generated by
  glow-discharge. Creation of glow-discharge
• Tube filled with argon, neutral, no charged
  particles between cathode and anode
• Elastic collision, no energy exchange
• Inelastic collision
• Energy not enough high, excite electrons,
  emitting photons
• Energy high enough, ionize electrons, cause
  secondary free electrons
• Both free electrons are accelerated again, so
  cause cascading or gas break down
• Flow of electrons is collected by anode, current
  will quickly decay to zero

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GLOW DISCHARGE PROCESSES
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Atoms into gas state

• at target
• target atoms ejected
• target ions ejected (1 - 2 )
• electrons emitted
• helps keep plasma going
• Ar ions reflected as Ar neutrals
• Ar buried in target
• photons emitted

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Sputter Deposition
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Fundamentals of Sputter Deposition
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Sputter Deposition
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Sputtering process

• momentum transfer process
• involves top 10 Å
• model as hard sphere collisions
• good for energies lt 50 keV
• 95 of incident energy goes into target
• gt COOL the target
• 5 of incident energy is carried off by target
  atoms
• typical energies of 5-100 eV
• target atoms come off with a non-uniform
  distribution
• more atoms normal to the surface
• cosine distribution (like surface source)
• characterize process by sputter yield (S)
• S number ejected / number incident

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• S depends on
• target material
• binding energy
• mass of atoms
• sputtering gas
• mass of atoms (S increases for heavier gasses)
• incident energy (S increases for higher energies)
• geometry
• most efficient 20-30 degrees from glancing
• for normal incidence sputtering
• maximum around 10 kV
• sputtering threshold
• S is about 1-10 typically

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• For calculating S we need
• number of atoms ejected
• depends on momentum and energy transferred
• these depend on relative masses and collision
  angle
• maximum energy transferred to target atom in hard
  sphere collision
• depends on binding energy of target atom
• number of layers involved in process
• mean free path of ion in target
• typically about 2 layers
• surface density of target atoms

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PVD Sputtering Tool
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PVD by Sputtering

• Sputtering Sputtering is a process whereby
  coating material is dislodged and ejected from
  the solid surface due to the momentum exchange
  associated with surface bombardment by energetic
  particles. Processes
• Gas ions are accelerated by a high voltage,
  producing a glow discharge or plasma
• A source (the cathode, also called the target )
  is bombarded in high vacuum by gas ions
• Atoms from the target are ejected by momentum
  transfer and move across the vacuum chamber
• Atoms are deposited on the substrate to be coated
  and form a thin film.

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Sputtering Mechanism
The kinetic energy of the impinging particles
largely dictates what event will happen

• Bounce back
• when very low energy (lt5eV) when the collision
  is head-on or nearly so
• Embedded
• when much higher energy (gt10KeV), the impinging
  particles are most likely to be embedded in the
  target, which is the basis of Ion Implantation
• If energy level is between the two extremes
• some fraction of the energy of the impinging ions
  is transferred to the solid in the form of heat,
  and lattice damage
• another fraction of such energy causes atoms from
  the surface to be dislodged and ejected into the
  gas phase----Sputtering

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Unique Characteristics

• Uniform thickness over large area.
• Simple thickness control.
• The alloy composition maintains stoichiometry
  with the original target composition.
• Deposition rates do not differ a great deal from
  one material to another.
• Sputtering-cleaning prior to initiating film
  deposition. The surface is not again exposed to
  ambient after such cleaning.
• The lifetime of a sputtering target may be as
  long as hundreds of runs and is seldom less than
  20.

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Coating Materials

• Metals Al, Cu, Zn, Au, Ni, Cr, W, Mo, Ti Alloys
  Ag-Cu, Pb-Sn, Al-Zn, Ni-Cr
• Nonmetals graphite, MoS2, WS2, PTFE
• Refractory oxides Al2O3, Cr2O3, Al2O3-Cr2O3,
  SiO2, ZrO2-Y2O3
• Refractory carbides TiC, ZrC, HfC, NbC, Tic-Ni,
  Tic-ZrC
• Refactory nitrides TiN, Ti2N, ZrN, HfN, TiN-ZrN,
  TiN-AlN-ZrN
• Refractory borides TiB2, ZrB2, HfB2, CrB2, MoB2
• Refactory silicides MoSi2, WSi2, Cr3Si2

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Thin film sputtering lines
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6 chamber thin film sputtering line
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Sputtering System
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DC sputtering
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RF Sputter Deposition
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Cathode sputter arrangement
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Cathode planar sputtering system
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• Sputtering alloy targets
• composition of alloy in film is approximately the
  same as alloy in target (unlike evaporation)
• rapid mixing in liquids (evaporation)
• slow diffusion mixing in solids (sputtering)
• target reaches steady state
• surface composition balances sputter yield
• Process
• Initial alloy of A and B .........................
  . ABABABABABABAB
• If SA gt SB , remove more A
• enriches surface in B .................... . BAB.
  B. BA. B . AB
• More B on surface gt more B sputtered .......
  ABABBABABBAB
• surface composition reaches steady state
• surface enriched in B
• bulk composition sputtered
• fASA / CA fBSB / CB
• where f surface fraction and C bulk
  composition
• alloy targets need to be conditioned by
  sputtering a few hundred Å before depositing

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• 2. Transport to substrate
• Target atoms pass through Ar gas and plasma
  environment
• one Ar ion for every 10,000 Ar neutrals
• electrons in plasma collide with Ar neutrals to
  form ions and more electrons
• Target atoms collide with Ar atoms, Ar ions and
  electrons
• treat as random walk "diffusion" through gas
• target atoms lose energy (down to 1-10 eV)
• chemical reactions may occur in gas
• not a line of sight process (unless pressure
  reduced)
• can coat around corners

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• 3. Deposit on substrate
• target atoms and ions impinge
• electrons impinge
• Ar atoms impinge
• Ar pressure about 0.1 torr
• Ar may be incorporated into film
• energetic particles may modify growth
• substrates heat up
• 100 - 200 C is common
• for a thermally isolated sample (no heat
  conduction)

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Parameters Argon Pressure
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• optimum deposition rate around 100 mTorr
• compromise between
• increasing number of Ar ions
• increasing scattering of Ar ions with neutral Ar
  atoms
• if you can increase the number of ions without
  increasing the number of neutrals, you can
  operate at lower pressures
• Sputter voltage
• maximize sputter yield (S)
• typically -2 to -5 kV
• Substrate Bias Voltage
• substrate is being bombarded by electrons and
  ions from target and plasma
• sputtering film while you deposit
• neutral atoms deposit independently
• put negative bias on the substrate to control
  this
• can significantly change film properties
• Substrate temperature
• control with substrate heater
• heating from deposited material
• increases with increasing sputter voltage
• decreases with increasing substrate bias

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Good for insulating materials in DC systems,
positive charge builds up on the cathode (target)
need 1012 volts to sputter insulators !! avoid
charge build up by alternating potential

• . . . . . . . . . . . . . . . . TIME -------gt
• sputter deposition occurs when target is negative
• substrate and chamber make a very large electrode
  - so not much sputtering of substrate
• Physical process
• frequencies less than about 50 kHz
• electrons and ions in plasma are mobile
• both follow the switching of the anode and
  cathode
• basically DC sputtering of both surfaces
• frequencies above about 50 kHz
• ions (heavy) can no longer follow the switching
• electrons can neutralize positive charge build up

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Advantages Easier to keep plasma going under
these conditions Can operate at lower Ar
pressures (1-15 mTorr) fewer gas collisions gt
more line of sight deposition
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Magnetron Sputter Deposition use with DC or
RF goal increase ionization of Ar Why? Higher
sputter rates at lower Ar pressures (down to 0.5
mTorr) fewer gas collisions - more line of
sight How ? increase probability of electrons
striking Ar increase electron path length use
electric and magnetic fields Most common
configuration crossed electric and magnetic
fields Put magnets (200 Gauss) behind target
traps electrons near cathode more ionization
near cathodes (10x) fewer electrons reach
substrate (less heating)
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Ion assisted deposition
with evaporation or sputtering (or chemical
vapor deposition) bombard surface with ions not
necessarily same type as in film ions typically
NOT incorporated in film relatively low voltages
(50 - 300 eV) leads to physical
rearrangement local heating can change film
properties for better or worse disruption of
columnar growth requires about 20 eV of added
energy per depositing atom
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Reactive Sputter deposition
add reactive gas to chamber during deposition
(evaporation or sputtering) oxygen,
nitrogen chemical reaction takes place on
substrate and target can poison target if
chemical reactions are faster than sputter
rate adjust reactive gas flow to get good
stoichiometry without incorporating excess gas
into film
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