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Physics of Thin Films

PES 449 / PHYS 549


Sputter Deposition Techniques

Ohring: Chapter 3, section 7


Basic Techniques

  • DC (diode)
  • RF (radio frequency)
  • magnetron


DC sputtering

simplest - basically what we have talked about so far

schematic of DC sputtering

Parameters:

  • Argon Pressure
    • deposition rate vs. argon pressure
    • 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
  • Deposition rate
    • changes with Ar pressure
    • increases with sputter yield
      • usually increases with high voltage
  • Particle Energy
    • increases with increasing sputter voltage
    • decreases with increasing substrate bias
    • decreases with increasing Ar pressure


RF Sputter Deposition

schematic of RF sputtering

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

target

-

-

-

+

-

-

-

+

-

substrate + chamber

+

+

+

-

+

+

+

-

+

. . . . . . . . . . . . . . . . TIME ------->

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

    RF sputter process

Advantages:

Easier to keep plasma going under these conditions

Can operate at lower Ar pressures (1-15 mTorr)

fewer gas collisions => more line of sight deposition


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:

magnetron configuration

 

  • traps electrons near cathode
    • more ionization near cathods (10x)
    • fewer electrons reach substrate (less heating)


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


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


Comparison of evaporation and sputtering

EVAPORATION
SPUTTERING

low energy atoms

higher energy atoms

high vacuum path

  • few collisions
  • line of sight deposition
  • little gas in film

low vacuum, plasma path

  • many collisions
  • less line of sight deposition
  • gas in film

larger grain size

smaller grain size

fewer grain orientations

many grain orientations

poorer adhesion

better adhesion


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